The purpose of this study was to examine the effects of diazinon and acephate applications to ornamental trees on the breeding behaviour and productivity of the American robin and to measure the effect of repeated chlorpyrifos spraying on robin productivity. No cases of adult or juvenile mortality were recorded, and nestling production was not affected. Plasma cholinesterase (ChE) activity in females exposed to diazinon or acephate was significantly lower than that in controls. However, there were no pronounced changes in parental care. The absence of a significant impact on behaviour despite the marked change in plasma ChE levels may have been due to one of several factors, i.e. (1) the insecticide had not reached the brain 18 to 24 h after spraying, and (2) exposure was insufficient to produce behavioural changes. The use of diazinon and acephate on ornamental trees should not be harmful to robins. There was a negative correlation between nest productivity and the number of lawns surrounding the nest treated with chlorpyrifos once or more in the preceding 2 years. The decline in earthworm numbers and their reduced biomass due to repeated chlorpyrifos use may account for the lower productivity recorded during the nesting season.