Thirty-nine isolates of Fusarium verticillioides from maize seeds from the three regions of Costa Rica were classified on fertility, fumonisin production, vegetative compatibility and pathogenecity. The identity of the isolates was verified by sexual crosses with standard tester strains and by isozyme analysis. Twenty-three isolates (59%) were mating type A- and 16 (41%) were A+; 29 (74%) were female fertile. The isolates produced high amounts of fumonisin B1 when grown on sterilized maize grits, 32 isolates producing more than 1000 µg g-1, as determined by TLC, and 7 less than 1000 µg g-1. Vegetative compatibility tests by pairing nit mutants identified 34 vegetative compatibility groups (VCGs), of which 29 had one member and 5 had two members. Isolates belonging to the same VCG were obtained from the same seed sample. Two pathogenicity tests with different inoculation methods were performed: on toothpick inoculation of 7-week-old maize stalks, 71% of the isolates were pathogenic according to the length of the necrosis formed in the stalk, and on sand inoculation of maize seedlings all the isolates were pathogenic, according to shoot length and dry weight production. Differences in aggressiveness between some of the isolates were recorded. It is concluded that natural populations of F. verticillioides in Costa Rica consist of genetically diverse, highly fertile and pathogenic isolates that represent a potential risk for disease development and fumonisin accumulation in maize crops.