To assess the role that temperature may play in determining the life cycle of Mysis relicta, we cultured newly hatched juveniles at 4 C and 8 C in the presence of a surfeit of food. Mysids first reached sexual maturity after 8 months at 8 C after molting 12–13 times. Mysids cultured at 4 C molted four to five fewer times in the same 8 months, yet grew the same amount per molt as the mysids cultured at 8 C. We predict that mysids that live in water of at least 8 C during the summer of their first year will complete their life cycle in 1 year, whereas mysids growing in water colder than 4 C during their first summer will require 2 years to complete their life cycle.