The carrier status and possible elevated tolerance of the signal crayfish (Pacifastacus leniusculus) against the crayfish plague disease agent (Aphanomyces astaci) has been speculated for several decades. We studied experimentally the responses of the signal crayfish from two Finnish populations against the crayfish plague infection by using two crayfish plague isolates with differing virulences; the highly virulent PsI-isolate and the mid virulent As-isolate. The studied signal crayfish came from two origins: the wild Leppavirta population and the Raukola farmed population of mixed genetic background. Both test groups were infected with A. astaci isolates using the challenge of 10 000 spores mL(-1). The isolates were PsI-Puujarvi (UEF8866-2) from Lake Puujarvi signal crayfish (year 2003) and As-Kivesjarvi (Evira6462/06) from Lake Kivesjarvi noble crayfish (year 2005 epidemic) in Finland. The crayfish were sampled prior to the experimental challenge with A. astaci to evaluate their A. astaci carrier status. The mortality, molting, feeding activity and other behavior of the crayfish were followed during the experiment. We observed increased mortality of the infected groups, related to the molting in some cases, and delayed molting. The impacts of the intense crayfish plague infection on the signal crayfish were clear and the studied populations showed differences in their reactions that are potentially explained by differing physiological conditions and different molt cycle phase. The results show that infection of both tested crayfish plague genotypes affects signal crayfish and is likely one of the main reasons behind losses among signal crayfish populations.