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When it comes to growth investing, market opportunity is a particularly important metric. The size of the market in which a business competes -- both now and in the future -- will ultimately determine how large a company can hope to become. And investing in a company that's early in its growth curve can lead to years of handsome profits as it fulfills its growth potential.

Today, we have one such opportunity in WisdomTree Investments (WT 1.07%), an excellent business with huge runways for growth still ahead.

Image source: WisdomTree Investments.

The mutual fund industry is a massive market, with nearly $16 trillion in assets here in the U.S., according to the Investment Company Institute. Yet it's also an industry in a state of transition, with hundreds of billions of dollars flowing into exchange-traded funds (ETFs) in recent years. 

ETFs have exploded in popularity since the first exchange-traded fund, the S&P 500 Index ETF (SPY -0.32%), was launched in 1993. By 2011, assets invested in ETFs reached $1 trillion, and they stand at more than $2 trillion today.

Helping to fuel this torrid growth are ETFs' low expense ratios, greater tax efficiency, and simpler trading mechanics compared to traditional mutual funds. Moreover, these valuable features are likely to continue to drive assets toward ETFs in the years and decades ahead.

Investors wishing to profit from this megatrend should consider WisdomTree Investments. As the only publicly traded asset manager exclusively focused on creating and managing ETFs, WisdomTree offers investors a pure play on the industry's staggering growth potential.

Even better, WisdomTree is benefiting not just from the overall growth of the ETF industry, but also from its lucrative niche as a leading provider of fundamentally weighted ETFs. Rather than simply mimicking an index like the market-cap weighted S&P 500 -- in which the largest businesses have the heaviest weighting -- WisdomTree's stock ETFs are designed to weigh metrics such as earnings and dividends. Fools can certainly appreciate this fundamentals-based approach, and judging by WisdomTree's impressive growth in assets under management, so too have many other investors.

WisdomTree launched its first ETFs in June 2006, and in the years since, it has become the ninth-largest provider of exchange traded products in the world, with more than $39 billion in AUM. The company's product lineup now includes 99 distinct ETFs that span many different countries and asset classes, including domestic and international stocks, fixed income, currencies, commodities, and alternative strategies.

Yet at its current size, WisdomTree still accounts for less than 2% of the ETF industry's total assets under management. And even after decades of rapid growth, the ETF industry as a whole is still dwarfed by the mutual fund industry. These facts should appeal to growth investors since they highlight WisdomTree's potential for exponential growth in the years to come.

It should be noted, though, that WisdomTree has suffered net outflows in recent quarters. This is mostly because of the company's highly successful Japan and Europe hedged equity ETFs, which have seen their popularity wane as Japan's economy has slowed, and as the lead up to and aftermath of the Brexit vote sapped investor confidence in the region. Yet with the majority of WisdomTree's products performing well, I see these outflows as a temporary situation. What's more, they're giving long-term investors the opportunity to buy WisdomTree's shares at a substantial discount, with the stock down more than 50% from its 52-week high in the wake of Brexit.

Looking ahead, fund flows in the WisdomTree International Hedged Equity ETF (HEDJ -1.15%) should improve as Brexit fears subside. In addition, the landslide victory for the Liberal Democratic Party in Japan's recent election is likely to be well received by investors in the WisdomTree Japan Hedged Equity ETF (DXJ 0.48%), as it ensures "Abenomics" -- economic stimulus measures championed by Prime Minister Shinzo Abe -- will continue.

Furthermore, new product introductions are diversifying WisdomTree's stable of funds, lessening the company's reliance on any one particular ETF. Most importantly, WisdomTree's innovative culture and fundamentals-based investment philosophy should continue to reward investors in both its ETFs and stock alike.

All told, WisdomTree is the best pure play on the rapid growth of the ETF industry -- and the massive, multitrillion-dollar addressable market for exchange-traded products worldwide.