Reproduction and colonizing behavior of pea aphids,Acyrthosiphon pisum(Harris), were studied on reproductive arrowleaf clover,Trifolium vesiculosumSavi, using four treatments: healthy, infected with bean yellow mosaic virus (BYMV), infected with aPhytophthorasp. that caused root rot, or infected with both BYMV and thePhytophthorasp. Reproduction of aphids in no-choice experiments was significantly lower on plants in the two treatments with the root rot disease than on healthy or BYMV-infected plants. The highest level of reproduction was observed on healthy plants. Aphid colonization also was lower on fungus-infected plants whether BYMV was present or not. Results of these studies suggest that a complex relationship exists between aphid vectors, virus disease, and fungal root disease in arrow leaf clover. Presence of fungal root rot disease may influence reproduction and movement of alate aphids and epidemiology of aphid-transmitted viruses during both fall and spring periods of alate aphid activity, when plants are in vegetative or reproductive stages, respectively.