SUMMARY.1. Mature crayfish, collected from an Irish lake before breeding had started, were held in breeding combinations and their mating and brooding activities observed.2. All mating attempts were initiated by the male. A single mating led to spawning within 6 days but a subsequent mating cancelled the effects of the first. Males mated more often when there were more females present. Males lacking a major cheliped mated less often than did normal males.3. Larger males mated more often than did smaller males, and although males showed no female size preference, matings were less frequent and generally unsuccessful when males were much larger than females; the female was usually killed. Large females mated successfully with smaller males.4. Females held at high densities with a larger male mated earlier than at low densities. However, aggression also increased with density; at high densities males fought and killed females.5. Males held in pairs without females fought; in occasional mating attempts spermatophores were not positioned correctly. Paired females rarely fought; all spawned normally although unmated. Although their eggs soon died and were removed during grooming, brooding behaviour continued for at least 2 months.6. Brooding females held in pairs shed pleopodal eggs during aggressive encounters. Females held singly showed a lower initial rate of egg loss.