SUMMARYMinimum and maximum net assimilation rates were determined from calculated curves of closest fit to data for 2 years. The minimum (14 g./m.2/week) occurred in mid‐December, immediately before the shortest day, and the maximum (78 g./m.2/week) between 27 June and 8 July, just after the longest day. The northern limit at which the tomato cannot maintain vegetative growth under glass in mid‐winter is probably latitude 59–60° and is determined by a limiting daylength of approximately 6 hr. The proportional reduction in net assimilation rate by shading was independent of the time of the year. The photosynthetic system of glasshouse tomato plants is not light‐saturated in midsummer in southern