Spirit Lake, Washington was radically altered limnologically by the May 1980 eruption of Mount St. Helens. The eruption provided a rare opportunity to study lake response and recovery in the wake of volcanic disturbance. During the eruption, and for several months thereafter, phytoplankton and zooplankton populations were subjected to extremely deleterious conditions. Consequently, these populations were virtually eliminated except for remnant organisms that somehow survived. During the next two years, the phytoplankton community and presumably the zooplankton community were comprised of only a few opportunistic species whose combined abundance was low. By 1983, however, phytoplankton abundance and species diversity had greatly increased due to increased lake-water transparency and increased availability of inorganic nitrogen. The reestablishment of the zooplankton community was also well underway by 1983, as indicated by the abundance of some species and the presence of most taxa that existed prior to the May 1980 eruption. By 1986, the phytoplankton and zooplankton communities were beginning to resemble those found in subalpine, oligotrophic/mesotrophic lakes in the Washington-Oregon Cascades. The rapid recovery of Spirit Lake demonstrated the vigor and resiliency of lake ecosystems and particularly plankton communities.