The diet and the temporal and spatial distribution of the freshwater crayfish Paranephrops zealandicus White were investigated in Lake Georgina, Canterbury. The digestive ability and efficiency of the crayfish were also examined in the laboratory. The diet of the crayfish during the period of the field study (January 1986 to November 1986) consisted largely of macrophyte detritus (principally Elodea canadensis), epilithic algae and exoskeletal material. Fish and other animal tissues appeared rarely in crayfish guts. Crayfish activity was investigated once every two months using a trapping programme. Activity was high in May and September and low in July. The September increase coincided with increases in moult frequency and egg production. Digestive enzyme activity was investigated using standard assay techniques. Hepatopancreas extracts containing digestive enzymes showed catalytic activity toward nine selected substrates. Microbial activity was implicated in the breakdown of three substrates, microcrystalline cellulose, laminarin and collagen. Ingestion rate and assimilation efficiency were affected by environmental temperature. At 15°C the rate of ingestion of fresh and decaying Elodea canadensis was highly variable and assimilation efficiency averaged 21%. At 5°C, feeding rate was greatly reduced but mean assimilation efficiency was 87%. Digestive enzyme activity also increased at low temperatures and may compensate for reduced ingestion to some extent. Crayfish size influenced ingestion rate but not assimilation efficiency. Sex and degree of decomposition of Elodea did not affect assimilation efficiency or ingestion. Temperature and photoperiod appear to have a strong influence on the crayfish population of Lake Georgina. They affected epilithic algal production and therefore provision of an important source of food, crayfish distribution, and both physical (i.e. ingestion and winter activity decrease) and metabolic (i.e. enzymic catalysis and assimilation efficiency) activity.