ABSTRACT The experiment is to study the effect of transplanting date and population of mustard (Brassica uncea) on yield of multiple cropping with plant tomato (Lycopersicum esculentun). The experiment had been carried out since September 29th until December 27th 1997 at Kepitu village, Trimulyo, Sleman, Yogyakarta. The soil type latosol with pH 6,5 and altitude 300 mete r s . …