Beware of those airline stocks.

Oof. 

Rising gas prices are starting to hit airlines. On Wednesday, May 30, airline stocks fell as oil topped $67 a barrel. Higher fuel prices could affect the ability for airlines to increase capacity in the short term, pointed out Delta Airlines at a conference on Wednesday.

The airlines index, which is shown below, shows how volatile the market has been for airline investors. In effect, the index has gone straight down since late February. 

When the market closed on Wednesday, American Airlines Group Inc. (AAL)  was down nearly 1%, Delta Air Lines Inc. (DAL - Get Report) was up slightly, JetBlue Airways Corp. (JBLU - Get Report) was down more than 1%, and Southwest Airlines Co. (LUV - Get Report) was down 0.3%.