Trials were conducted for three seasons at a range of altitudes in Tanzania to investigate the effect of plant populations and levels of applied nitrogen and phosphate on the performance of maize varieties. Increasing population to 47,500 plants per hectare gave increased yields even though cob size and number of cobs per 100 plants showed considerable reductions, but 72,000 plants/ha. is probably too many for late maturing varieties. Sites over 900 m. showed good response to applied fertilizer, but below 900 m. response was much smaller or absent. Cob height was increased by fertilizer but hardly affected by spacing, no treatment made any appreciable difference to days to 50 per cent pollen shed, and lodging increased with closer spacing.