A severe drought in 1977 resulted in cessation of normally permanent flow m a northern California (USA) springbrook, and no individuals of the numerically dominant macroinvertebrate, the caddisfly Gumaga nigricula (McL), survived Prior to habitat loss (1976) this population was multiple cohort, two years after habitat recovery (1979) a single-cohort population was present The shift m age structure back to pre-drought conditions occurred gradually median head capsule width in 1976 (pre-drought) 0 39 mm, 1981 (post-drought) 0 95 mm, 1982 0 99 mm, 1983 0 59 mm, 1984 0 63 mm, 1985 0 63 mm, 1986 0 45 mm Age structure from 1981-1985 was significantly different (p < 0.001) from that occurring before habitat loss in 1976, age structure m 1986 was not significantly different from that in 1976 The ten-year time for return to the original population age structure could be influenced by the isolation of the springbrook from other colonization sources, gradual increases in population density, and differential growth reflecting food availability