The genetic relatedness of twenty isolates of Paecilomyces farinosus was determined by comparison of the products of polymerase chain reaction amplification of anonymous regions of genomic DNA with single arbitrary sequence oligonucleotide primers (RAPD analysis). Isolates were collected from seven insect species in eastern Canada and they differed greatly in cultural and morphological phenotype. All P. farinosus isolates were clearly distinguished from three other entomopathogenic fungi, including P. fumosoroseus. RAPD banding patterns did not, however, correlate with ecological backgrounds or morphological phenotypes of P. farinosus isolates. These observations support the conclusion that P. farinosus from eastern Canada is not composed of strains which can be separated on the basis of the ecological or morphological criteria selected.