The last five years have witnessed a dramatic increase in interest in maize growing in New Zealand. As a result several new varieties which were developed under conditions of high plant population density in North America (Downey, 1971 c) have been released for commercial grain production in New Zealand. In addition it has been established that a number of districts previously regarded as marginal for the production of this crop are in fact quite suited to growing maize for grain. Because the optimum plant population for grain production is determined by the variety of maize planted and the environment in which it is grown (Dungan et al, 1958), it has become necessary to quantitatively evaluate the yield density relationship of commercial hybrids in the previously untested environments in which maize is now being grown. Although maize has been grown in New Zealand for many years, a number of fundamental agronomic questions have remained unanswered. Despite a considerable recent expansion in area grown for grain, the avarage yield of maize in New Zealand is more than one third greater than that in the United States (Statistical Abstract of the United States, 1970). There is a total absence of any agronomic information in New Zealand to account for this yield superiority of comparable genetic material when grown in this country. The present study was carried out in an effort to remedy some of these areas of deficiency. The stock commercial hybrid grown in New Zealand over the last decade, W575, was chosen as a full season variety, thereby providing a baseline against which the relatively unknown short season variety, JIC3, could be compared. This experiment was designed to provide basic agronomic information on the growth and yield of two varieties of contrasting maturities when grown over a range of commercial field spacings thought to be suitable for the Manawatu.