We investigated relationships between the diet and morphology of what appeared to be two undescribed species and intermediate morphs of the threespine sticklebackGasterosteus aculeatuscomplex living in a closed-basin, coastal lake in British Columbia during 1969 and 1970. One “species” was without a pelvic girdle and lateral plates and the other had a complete pelvic girdle, pelvic spines, and five or more lateral plates per side. For convenience, the former species was referred to as “benthic” because of its association with the lake bottom and the other as “limnetic” because of its pelagic distribution. The two species and intermediate morphs were divided into four groups based on girdle condition (none, partial, complete with incomplete pelvic spine development, and complete with long pelvic spines). Each group was divided into four subgroups based on lateral plate number (0, 1–2, 3–4, and 5 or more) on one side. Plateless morphs of each group fed mostly on bottom prey. Consumption of pelagic prey was associated with an increase in plate number for all groups in 1969. For a given plate number, however, the percentage of pelagic food in the diets was higher as girdle condition increased. In 1970, when there was intense predation on the threespine sticklebacks by introduced coho salmonOncorhynchus kisutchonly individuals in subgroups with complete girdles continued the trend of greater consumption of pelagic prey with an increase in plate number. Although the sample sizes were very small for some subgroups, the results suggested that higher numbers of lateral plates, in combination with girdle condition, were associated with pelagic feeding behavior of all groups in absence of coho salmon predation in 1969. In 1970, the pelvic girdle and lateral plate armor were important in maintaining pelagic feeding behavior in the presence of coho salmon predation.