Entomophaga maimaiga is a naturally occurring fungal pathogen specific to larvae of the gypsy moth, Lymantria dispar. E. maimaiga is thought to be native to Asia where its epizootics can suppress gypsy moth outbreaks. However, in the USA this beneficial fungal pathogen was not observed until 1989, although an isolate of E. maimaiga from Tokyo was released in Massachusetts to control gypsy moths as early as in 1910-1911, and another isolate from Ishikawa Prefecture in Japan was later released in 1985 and 1986 in New York and Virginia. Our objectives were to: (1) test the hypothesis that E. maimaiga populations in the USA have reduced genetic variability due to founder effects compared to the putative ancestral populations in Asia; (2) track the origin of the North American populations of this fungus; and (3) assess whether genetic differences among E. maimaiga isolates are correlated to morphological differences. We compared genetic diversity among 30 E. maimaiga isolates originating from seven states in the USA, five prefectures in Japan, one province of China and one region of far eastern Russia by AFLPs. Among 14 USA isolates, only ten polymorphic AFLP loci were found, whereas 56 polymorphic loci were found among 16 Asian isolates; 29 loci were polymorphic among 12 isolates from Japan alone. Average gene diversity (h) for the polymorphic loci was 0.223 +/- 0.005 for Asia (including Japan), 0.131 +/- 0.006 for Japan only, and 0.041 +/- 0.004 for the USA. Thus, native populations from Asia were more diverse than the USA populations. These results are consistent with the expectation of a population founded from a source population by a small number of individuals. Distance and parsimony analyses of AFLP data showed that the isolates from the USA formed one distinct clade that was most closely related to Japanese isolates collected outside the Tokyo area. No morphological variation of E. maimaiga from different geographical locations was detected.