Field experiments were conducted under arid zone conditions to examine cotton response to treated domestic wastewater applied at different frequencies and emitter spacings by trickle irrigation. Row spacing was either conventional, i.e. single rows apart, or in twin rows, 0.96 m and 1.92 m apart (a twin row consists of a pair 0.35 m apart). Some treatments received the commercial rates of effluent application (around 5,500 m3/ha) as commonly practiced in the area, others received a reduced amount, 80% of the commercial rate. Maximum yield (over 7,000 kg/ha) was obtained under twin row planting, irrigated twice a week with a commercial amount of effluent. A yield of 6,040 kg/ha was obtained under twin row planting irrigated once a week or each day. Under conventional row and emitter spacing, irrigated twice a week, the yield was 6.016 kg/ha.