Two greenhouse experiments were conducted to determine the interaction and population dynamics of the western flower thrips (WFT), Frankliniella occidentalis (Pergande), and the twospotted spider mite (TSM), Tetranychus urticae Koch, together or alone on impatiens (Impatiens wallerana Hook). In both experiments, single pest (control) treatments involved inoculating plants with 10 individuals of one species or the other. Two-pest treatments were inoculated with 5:5 or 10:10 (TSM:WFT). In the first experiment, only the cultivar ‘Impulse Orange’ was used. In the second experiment, we used ‘Impulse Orange’ and ‘Cajun Carmine’. In the second experiment, populations of WFT and TSM were both significantly lower on ‘Cajun Carmine’ than on ‘Impulse Orange’, indicating that ‘Cajun Carmine’ is a more resistant cultivar. However, in all tests WFT had a significantly negative effect on population growth of TSM while the presence of TSM did not influence population growth of WFT. WFT may negatively affect TSM population by both predation and competition. Predation by WFT on TSM eggs could influence the population dynamics of TSM, but preying on TSM eggs did not benefit the WFT. TSM and WFT might also interact indirectly through their common resource, which may be another factor influencing TSM population growth.