This paper is concerned with insect visitors to some Canadian weeds. It shows that both self-pollinating (autogamous) and cross-pollinating (allogamous) weeds are visited during the flowering period. It was found that self-incompatible species were visited by insects frequently, whereas self-compatible weeds were not visited or were rarely visited. The self-compatible group showed some difference, in that autogamous annual and winter annual weeds were visited less than were autogamous biennial and short-lived perennial weeds. Most of the insect visitors, to both the autogamous and allogamous species of weeds, were native species, whereas the weeds themselves were mostly introduced species. The insects, also, evidently visited more than a single species of plant over a short period of time. The implications of the insect visits to the genetic structure of the plants and their colonizing abilities are then briefly discussed.