Sampling with the Isaacs-Kidd midwater trawl yields good quantities of large and easily-identified animals that can be used as "indicator organisms" to identify water masses in the northeastern Pacific Ocean. Particularly useful are the myctophid fishes Lampanyctus leucopsarus, Diaphus theta, Tarletonbeania crenularis, Electrona arctica, E. crockeri, Ceratoscopelus townsendi and Lampanyctus ritteri, the pteropods Limacina helicina, Cavolinia globulosa and Euclio pyramidata, and the heteropod genus Atlanta. In going from Subarctic water to warmer Intermediate water in the open ocean, numbers of animals declined and the species changed markedly. By the time quantities of Central or Tropical water are encountered, catches were about [Formula: see text] of those in Subarctic water. Inshore catches off southern British Columbia and Washington in 1958 were only about [Formula: see text] as great as in comparable hauls in 1957 — a difference which coincides with a greater intrusion of Intermediate water in 1958.