Thirty-five maize seed samples from three geographic regions of Costa Rica were analysed for fumonisin content, germinability, and Fusarium verticillioides infection with and without surface disinfection. The concentration of fumonisins in the maize samples varied from 4 ng g-1 to 16 000 ng g-1 with an all over-all average of 2500 ng g-1. There was a significant difference in fumonisin content between samples from Alajuela and Guanacaste. Germination of the seed samples ranged from 12% to 98% with significant differences between regions. F. verticillioides infection was 12-98% and 13-97% for surface-disinfected and nondisinfected seeds, respectively. There was a significant negative correlation (r=-0·52) between fumonisin content and seed germination, but no significant correlation was found between fumonisin content and F. verticillioides infection, or between F. verticillioides infection and seed germination. Most of the high fumonisin seed samples had some mechanical or insect damage. Whether or not the fumonisins had a direct effect on germination was not further established. It is concluded that the large differences in fumonisin content of maize seeds within and between regions are primarily caused by differences in seed quality, genetic diversity of F. verticillioides strains in natural populations, climatic differences between regions, and varietal differences. Some of the fumonisin levels found in this study coincide with levels associated with risks to humans and animals in other countries.