The smolt stage of the life history of Pacific salmon (Onchorhynchus sp.) has received little attention relative to the other life history stages. I examined mortality during the downstream migration of coho salmon (0. kisutch) smolts by trapping and marking smolts high in the watershed and recapturing the fish near the ocean. Four study sites were used, two in the upper watershed and two in the lower watershed. At both distances, lake-type and stream-type habitats were sampled. Factors which were found to influence survival included: 1) distance of migration, lower survival with increased migration distance; 2) juvenile rearing habitat, stream-type fish had higher survival; and 3) the timing of migration, fish migrating early in the season survived better. A fourth factor examined, the length of the fish at migration, did not influence survival. Although length did not affect survival, it was found that the center of the length distribution often had higher survival than the tails of the distribution, regardless of the absolute length. I performed two separate experiments to assess the effects of handling on fish survival. These experiments did not indicate my handling the fish affected survival. This thesis establishes differential survival occurs in salmon smolts, however the mechanisms causing differential survival are not known.