They are probably Fungus Gnats. Are you perhaps keeping the soil a bit moist/wet? Those danged gnats are usually a nuisance when the potting medium is too wet. You can read tips for alleviating these gnats in the articles below which were written by some of our ATP members:
http://garden.org/search/index...
The first says to use a couple of drops of hydrogen peroxide in the water when watering plants but if the soil is already too wet you don't want it wetter. The second article is tips on alleviating a few different plant pests; scroll down to find the tip for Fungus Gnats.
I only had issues with those gnats one time, years ago when I was too lenient with my watering ... nowadays I'm just the opposite, my plants are all kept on the dry side. The only thing I use is Insecticidal Soap spray (found at Home Depot, Walmart, Garden Centers, etc.) which takes care of gnats and a lot of other pests.
