Because of the lack of reports,the base levels of microbial contamination on stored fuels are unknown in tropical regions and it is unclear whether these levels have some influence on fuel quality parameters.Therefore,fungal quality in automobile fuels stored across Costa Rican territory was evaluated during two years according to the standard ASTM D6974-04.For a total of 96 samples,counts and identification of molds and yeasts were performed on regular gas,premium gas and diesel taken from the bottom and superior part of the container tanks.The highest contamination was found on the bottom of the tanks,where an aqueous phase was usually identified,showing populations over the ones present in the hydrocarbon itself (up to 108 CFU/L).Diesel was the most contaminated fuel (up to 107 CFU/L);however,an alteration on the physicochemical parameters was not observed in any kind of fuel.Seventy-five mold strains were isolated,Penicillium sp.being the most common genus (45.8% of the samples),and ten yeast strains,from the genera Candida sp.and Rhodotorula sp.Four of the yeasts were able to grow on diesel as the sole carbon source,at concentrations ranging from 0.5% to 25%.Increasing the frequency of tank cleaning,adding antimicrobial agents and monitoring microbial populations are recommended strategies to improve microbial quality of stored fuels.