Two K-field experiments were conducted at Sakha Agric Res. St., Kafr El-Sheikh Governorate during 1993 and 1994 seasons to study the response of Giza 77 cotton cultivar to foliar and soil applications of po­tassium sulphate and its timing. The treatments included unfertilized check, soil application; before sowing, after thinning, and at early bloom­ing, foliar feeding 2 weeks after early bloom, soil application after thin­ning + foliar feeding 2 weeks after early bloom. Soil application was done using 24 kg K20/fed., while foliar feeding was carried out with K2SO4 solution (1%K20). Results indicated the positive effect of potassium sulphate when it was applied at the proper time as soil application after thinning which resulted in taller plants with higher number of fruiting branches, less number of vegetative branches, higher boll retention, heavier bolls as well as high seed cotton yield per plant and per feddan and lint percent­age. The benefit from foliar application of K-solution 2 weeks after 1st bloom was realized, specially when it was applied as a supplementary feeding combined with soil application after thinning. Potassium fertiliza­tion showed no effect on seed index.