The chemical and physical compatibilities of selected insecticides co-applied with a 22% urea foliar fertilizer were tested. No degradation of (14)C-labeled lambdacyhalothrin (technical grade) was detected by thin-layer chromatography, but the partitioning behavior of this pyrethroid was affected by the urea fertilizer. Distribution patterns of the insecticide in solutions revealed that urea disrupted the formation of an emulsion. In tests measuring the effects of urea on the distribution of lambda-cyhalothrin, significantly more insecticide was found in the top one-third of the solution compared to that in the bottom one-third. After five minutes, 46% of the radiolabeled insecticide was found in the top layer of the water solution whereas 69% was found in the top layer of the urea fertilizer. Similar tests revealed that insecticide distribution in solution was directly affected by the concentration of urea. A significant correlation was found between turbidity and the amount of insecticide in solution. Turbidity tests indicated that the distribution of other selected insecticides in a 22% urea fertilizer was adversely affected. The use of radiolabeled Karate disclosed that none of seven adjuvants tested corrected the compatibility problem at its recommended rate.