Populations of the western flower thrips, Frankliniella occidentalis (Pergande), were observed in field plots of cotton, Gossypium Hirsutum L., Deltapine-61 (DPL-61) and WC-12NL, a nectariless, okraleaf variety in the DES-56 germplasm line, and fields of Stoneville-825, DPL-61, and DPL-77 in Maricopa, AZ, during June-August, 1987. Square shed was observed on the upper 5 nodes of plants, and the bractedness (3 bracts = normal, more than 3 bracts = abnormal) of matchhead size squares was determined from samples of squares picked from the plants and from shed squares beneath the plants. Shed squares were dissected to determine the cause of shedding. The percent of normal 3-bract squares on the plants was significantly greater for WC-12NL than for DPL-61 (99 compared to 90%, respectively) and 3-bract squares were found in the same percentages among shed squares. In shed squares, the 3-bract squares had greater shed due to physiological stress and less shed due to feeding by thrips than abnormal squares. Among the 3 varieties in fields, ST825 had significantly fewer shed squares (6%) than DPL61 (17%) or DPL-77 (20%) but all had comparable populations of thrips (average of 5/terminal). There were no differences among the percentage of squares shed due to physiological stress (average of 40%). The percentage of 3-bract squares on plants was greatest for ST-825 (97%) and lower for DPL-61 (90%) and DPL-77 (79%). The percentage of 3-bract squares in samples of shed squares were virtually identical to those found on the plants. Shedding due to thrips was significantly greater in abnormal squares (average of 62% for all fields) than 3-bract squares (average of 28%). These results indicate that thrips were the primary cause of shedding of abnormal squares and a secondary cause of shedding in 3-bract squares.