The Financial Times reported earlier this week that Citigroup (C -0.32%) has told investors it expected a "significant drop in trading revenue" this quarter, but it shouldn't be the only bank to feel the pain of falling revenues.

According to the report, "Those familiar with Citi's discussions with its investors said the bank had highlighted a marketwide slowdown in activity. Some investors believe revenues will fall by significantly more than 10 percent."

Source: Baer Tierkel.

This news comes less than one week after Jefferies Group, an investment bank owned by Leucadia National (JEF 0.02%), reported that its profit plummeted from roughly $70 million in the third quarter of last year to $12 million this year. This was driven by its fixed-income trading unit watching its revenue fall 88% year over year to $33 million. Ouch.

Its trading revenue stood at its lowest level since the depths of the financial crisis, the fourth quarter of 2008.

Of the four largest banks in the U.S., three are likely nervous about what the future may hold if revenue from trading continues to fall. Let's check in on the percentage of the revenue that comes from trading, particularly fixed income, to see which banks could be most at risk.

Source: Company SEC filing.

In 2012, Citigroup had approximately 22% of its $75 billion in revenue come from trading activities. While this is certainly the most of the four largest banks in the U.S., roughly 90% of its trading revenue (or 20% of its overall revenue) came from its fixed-income business. This trend held true through the first six months of 2013, as it again had its fixed-income revenue make up 20% of its overall revenue.

Source: Company Earnings Reports

Like Citigroup, JPMorgan is heavily dependent on its trading business to drive revenue.However, it is has a slightly better balance between its equity and fixed income divisions.

In 2012, about 16% of its total revenue came from its fixed income team, but that number did creep upwards to 17.5% through the first six months of the year. JPMorgan is likely on its best behavior following the London Whale fiasco (which did not occur in the trading operations), so it will be curious to watch how its revenue shifts as it avoids any major potential losses as a result of the decline in bond trading.

Source: Company SEC filings.

Next on the list is Bank of America -- which has also watched its trading revenue as a percent of total revenue creep upward through the first six months of 2013 compared to last year (16.5% versus 14%).

Yet of the three, it has seemingly the best balance between fixed-income and equity trading, as its fixed-income line item also encompasses currency and commodities trading, which represented 75% of total trading revenue.

Source: Company SEC filings.

Unsurprisingly, Wells Fargo in facing the least amount of risk, as any potential drop in trading would have an almost unnoticeable impact to its bottom line.

In fact, in its most recent 43-page earnings release, it never even mentions "fixed income." While the bank faces potentially sharply lower mortgage-related revenue, shareholders of Wells Fargo can sleep easy tonight as they consider any risk of the fall in bond trading.