The relationship between embryo development, expressed as cumulative temperature units (TU; the number of TU is the difference between the daily mean temperature and 0 C), and the timing of the period during which embryos are sensitive to handling was determined for coho salmon (Oncorhynchus kisutch) and rainbow trout (O. mykiss, formerly Salmo gairdneri). Each day after fertilization, a different group of embryos was subjected to standardized handling stress; subsequent survival in the group was compared to that of unhandled controls. Both species were incubated at three temperatures. The sensitive developmental stages (when handling significantly lowered survival; P 0.05) for coho salmon embryos was 90-139 TU at a mean incubation temperature of 8.9 C, 95-145 TU at 10.5 C, and 84-124 TU at 12.7 C. Minimum survival (about 20%) occurred at 111, 115, and 98 TU for temperatures of 8.9, 10.5, and 12.7 C, respectively. Rainbow trout embryos survived significantly less well when handled at 99 and 98 TU for incubation temperatures of 9.3 and 10.4 C, respectively, but they evidenced no sensitivity to handling when incubated at 11.5 C.