||Diagnostic work was undertaken during 1987-91 to determine the area of maize planted late by smallholder farmers in sub-humid (Natural Regions 2 and 3) communal areas of Zimbabwe, the grain yield losses encountered, and the causes of the low yield. From a survey of 10 communal areas in 1989-90 it was estimated that smallholders in Natural Regions 2 and 3 planted around 175,000 ha of maize four ou more weeks after the start of maize plantings. This represents some 32.9% of the total maize area planted in sub-humid communal areas each year. From detailed agronomic monitoring in Mangwende Communal Area during 1987-88 and 1988-89, the first quartile of fields planted (mean planting date 14 November) averaged a grain yield of 4.9 t ha against only 1.1 t ha for the last quartile of fields planted (mean of 22 December). This was equivalent to around 310,000 t of maize grain foregone each year in sub-humid parts of Zimbabwe because of late planting. Observations on farmers' fields in Mangwende, confirmed in a on-station experiment at Harare, showed that the early plantings, with two to three more emerged leaves at any time up to five fewer days to tassel emergence. Many minor and several major input and management differences were noted for late plantings versus early plantings. Input and management differences likely to be major contributors to the low grain yield of late planted maize were: 1) delayed application of basal fertilizer (by around six days, three leaves later) at a reduced rate (around 20 kg ha N, 15 kg ha P only); 2) topdress N fertilizer was applied at just 60% of the rate given to early plantings, and two leaves later in crop development; and 3) the late planted crop suffered weed competition longer than did early plantings, with greater weed burden at planting, and was first weeded at the -ñeaf stage (four leaves later than with first plantings). The diagnostic results reported here proved useful in planning a research project to generate technologies to improve the productivity of late planted maize in Zimbabwe.