Regular research vessel surveys in Trinity Bay, Nfld., during 1967–68 indicated that significant biological changes occurred in coastal capelin from the over-wintering to the spawning period. During their over-wintering period in February–March mature capelin had a high fat content, a low feeding intensity, a low nematode incidence, and were heavy at length. The gonads were in a resting stage, secondary sexual characteristics were absent, and the sexes were integrated. During the prespawning period in April–May the feeding intensity increased and the gonads and secondary sexual characteristics began to develop, whereas the fat content declined. At this time the schools were segregated by sex. By the start of the spawning season in mid-June the capelin had ceased feeding, the fat content was low, and the weight at any particular length was less than that during the over-wintering period. The incidence of nematodes was high and the sexes were readily distinguished by the conspicuous secondary sexual characteristics. These changes are discussed in relation to the seasonal distribution pattern of coastal capelin in Trinity Bay and the feasibility of a commercial fishery.