Evidence from published trials is reviewed to investigate whether there is any need to change the recommended optimal light intensity for laying fowls from 5 lux. Although some recent trials have recorded excellent performances from birds exposed to light intensities of less than 5 lux, it is concluded that this has more to do with the ability of modern pullets to lay well even without photostimulation than with any change in the threshold of light sensitivity. There is good recent evidence that the stimulation of early sexual maturity a large increment in the photoperiod during the growing stage requires minimal intensity of 2lux, and that 41ux is probably required for maximal response. By combining the results from all available trials, it is possible demonstrate small but significant effects of light intensity on egg size, food intake and mortality. An economic analysis, which takes into account all these responses, indicates that the optimal profit occurs at about .5 lux. Although the prices used in that analysis are both local and ephemeral, the optimum will vary very little within other cost structures, because it is determined almost entirely by the lower outputs predicted at intensities below 51ux and the greater electricity cost in providing very much higher intensities. It recognised that, because maximising profit is not always the sole objective enterprises involving animal production, considerations for the welfare of both the livestock and workers may make it prudent to recommend a slightly higher value such as 1O lux.