Crayfish plague, caused by the oomycete Aphanomyces astaci, was first diagnosed in Irish stocks of Austropotamobius pallipes from a midlands limestone lake and crayfish farm in October, 1987. Behavioural activity of infected crayfish was monitored and the position at death noted. Crayfish showed no gross, clinical or behavioural signs although they were somewhat lethargic for about 14 days after infection before a rapid deterioration in their condition. Death followed after about 18 and 21 days at 10 ° and 5 °C respectively, with approximately 85% of the animals dying in the open. Spores remained viable and infective in tanks for between 6 and 9 days after death of an infected crayfish at 10 °C. Based on these experiments, suggestions are given for containing the Irish plague outbreak.