Abstract: Experiment was carried out to investigate the effect of cephalexin on sex expression, fruit setting, fruit development and yield of cucumber. Cucumber was grown in the field at natural condition and treated with cephalexin solutions of 0,12.5, 25, 50, 75 and 100 ppm at seedling stage (2-true leaf stage). Data were collected on number of male flowers/plant, number of female flowers/plant, number of fruits/plant, days to first male flowers, days to first female flowers, % fruit setting, fruit size, fruit weight and fruit yield/plant. The results revealed that foliar application of cephalexin changed the growth habit of cucumber reducing the vegetative phase and extending flowering phase of the plant. The lowest vegetative phase (30.3 days) and the highest reproductive phase (65 days) was found in the plants treated with 50 ppm cephalexin. It suppressed the production of male flowers but increased the number of female flowers thereby subsiding the ratio of male to female flowers. Cephalexin significantly increased the fruit setting over control with the highest (68.7%) in 50 ppm. It also increased the fruit length, girth, weight and ultimately total yield/plant by 48.0, 55.0, 52.6 and 172.2%, respectively, in 50 ppm treatment.