Adult squash bugs, Anasa tristis (De Geer) (Heteroptera: Coreidae), were confined on watermelon plants at differing phenological stages and at densities of zero to four per plant in one trial and zero to 32 per plant in three additional trials. Squash bugs were allowed to feed on the plants until plants died or fruit matured. Plant foliage, roots, and fruit were harvested and weighed to determine effects on growth and productivity. Growth and fruit production was regressed on number of squash bugs and results indicated that an increasing density of squash bugs feeding on vining and flowering stage plants resulted in significant reductions in plant growth and fruit yield. When plants were infested at the fruit set stage of growth, there was either less effect or no effect on plant growth and fruit production. Plant mortality increased as the density of squash bugs increased regardless of the stage of growth when plants were infested with squash bugs.