An investigation of the toxic effects from single concentrated doses of 1-naphthyl (hydroxymethyl) carbamate, 1,5-dihydroxynaphthalene, or 1,3,5-trichlorobenzene demonstrated a marked increase in death of the embryos in eggs deposited from the 7th to 14th days after treatment ofBracon hebetorSay virgin females. Hatchability returned to control levels about the 15th day. Associated with poor hatchability was an increase in the proportion of embryos dying during cleavage (Stage 1 Death). This indicated the vulnerable cells of the ovariole sequence to be those undergoing mitosis, a finding consistent with the reports of damage to the mitotic apparatus by related compounds in other organisms. Egg production was decreased only slightly. The results were similar whether the females were injected with 1 of the agents or exposed to a residual deposit of it.