ABSTRACT: Chitinase, as determined by enzymatic activity in the growth medium and by transcription of the chitinase gene AaChi1, is expressed at a high level during vegetative growth of the crayfish pathogen Aphanomyces astaci and expression is not further stimulated by chitin. Expression is not detected in zoospores and it does not increase to high levels until late during germination. Upon sporulation, chitinase expression increases to levels comparable with those seen in fast-growing mycelium. This pattern of chitinase expression is a common feature of strains representing all currently known genotypes of A. astaci, suggesting that it is an adaptation to the exclusively parasitic life-style of this species. In contrast, other Aphanomyces spp. including the saprophytes, A. laevis and A. stellatus, produce significant amounts of chitinase only in the presence of chitin. The pattern of chitinase expression is one of very few qualitative physiological characteristics known which can distinguish A. astaci from other parasitic and saprophytic species and may thus be of practical use for identification.