Interspecific interactions between Bemisia tabaci (Gennadius) and the spidermite Tetranychus truncatus Ehara were evaluated by observation of behavioral responses over 30 min intervals on cowpea leaves. The response of adult female whiteflies was observed on 4 classes of leaf: (1) uninfested (2) previously fed upon by spidermite nymphs, (3) spidermite nymphs and webbing present, and (4) spidermite adults present. Likewise, adult female spidermite response was observed on (1) leaves uninfested with whiteflies, (2) leaves previously fed upon by whitefly nymphs, (3) leaves with whitefly nymphs present, and (4) leaves with whitefly adults present. Spidermites generally had a greater effect on whitefly behavior than vice versa. Whitefly feeding duration decreased while labial tapping duration and frequency increased on all 3 types of mite-infested leaves compared to uninfested leaves. Although whitefly oviposition duration and frequency did not change on leaves infested with spidermite nymphs, whitefly fecundity decreased compared to untreated leaves. Although whitefly effects on spidermite behavior were less marked, nevertheless, duration and frequency of cheliceral tapping by spidermites did increase in the presence of whiteflies, although duration or frequency of feeding did not. Thus, one would predict that spidermites would win out in competition with whiteflies on a common host plant. Behavioral observations of interactions between herbivores can provide valuable insight into competitive relationships which should, however, be verified by field studies.