The acute toxicity of cadmium to three larval instars (1 st, 3rd and 4th) of the cased trichopteran Agapetus fuscipes Curtis was determined for animals both in the cased and uncased state. First instar larvae were significantly more sensitive than 3rd or 4th instars and for all ages cased animals were more resistant than uncased. Sub-lethal toxicity was demonstrated by a reduction in case building activity and aggregation response. The need to consider life cycle information when designing toxicity tests and the importance of incorporating sub-lethal responses into a study are discussed.