Life table studies were conducted on both sexes of newly emerged subimagoes of the mayfly species, Hexagenia limbata (n = 4472) and Ephemera simulans (n = 1254), which have two- and one-year life cycles, respectively. The expectation of life at emergence for H. limbata was approximately 2.6 days for both sexes with the last individual (a female) dying on day 8. Expectation of life for E. simulans was 1.6 and 2.0 days for males and females, respectively, with the last individuals (both sexes) dying on day 3. The results reveal that mortality does not occur in one large step in this group of rapidly senescing organisms. Rather deaths are spread over several days when the adult mayflies are maintained under laboratory conditions.