Clean Energy Fuels (Nasdaq: CLNE), a provider of natural gas fueling stations, is widely viewed as the main beneficiary of an emerging natural gas vehicle market in the U.S.

But what happens to Clean Energy Fuels if the utility companies or, heaven forbid, Big Oil want a piece of the action? Well, we might find out.

Piedmont Natural Gas (NYSE: PNY) plans to build a network of natural gas fueling stations across the Carolinas and Tennessee. The natural gas utility company is taking a page from Clean Energy's playbook and targeting business fleets as the primary customers for the fueling stations.

Other natural gas utility companies are entering the market as well.

Atlanta Gas Light, a subsidiary of AGL Resources (NYSE: AGL), is seeking state funds to construct a network of 10 to 15 natural gas filling stations in the Atlanta metro area. The move is prompted, in part, by the success of Clean Energy Fuels' CNG stations in the area.

And that's just the utility companies. The biggest competitive threat to Clean Energy Fuels may come from the major integrated oil companies.

Chevron (NYSE: CVX) offers CNG alongside gasoline and diesel at fueling stations in Pakistan. Royal Dutch Shell (NYSE: RDS-B) plans to do the same in Canada. If natural gas transportation catches on in the States, Big Oil could simply domesticate its foreign fueling station strategy.

Clean Energy Fuels offers an unattractive risk/reward profile because of the competitive environment. The other natural gas transportation stocks look to be a better way to invest in this trend.

Fool contributor Adam J. Crawford does not own shares in any company mentioned in this article. Motley Fool newsletter services have recommended buying shares of Chevron and Piedmont Natural Gas. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.