Chaetosiphon fragaefolii(Cockerell) was studied to determine the factors regulating its populations. Fertilizers, irrigation, defoliation to stimulate production of new leaves, and removal of blossoms had only temporary effects or none on populations. Aphid populations were much larger on plants covered by cages in the field, indicating that predators were an important controlling factor. However, parasitized aphids were never found. Populations became even larger in an unheated greenhouse and remained large through the winter, even at subfreezing temperatures, whereas populations became low in field cages. Wind and rain probably are detrimental to the aphids. The field population always declined in the late summer, even in cages. Analysis of strawberry foliage showed that populations of this aphid tend to correlate with the concentration of nonreducing sugars (probably mostly sucrose). Decline in nutritional value of the leaves coupled with increasing predator populations are probably the prime reasons for the rapid drop in population size in mid- to late summer each year.