FIGURES obtained by catching insects in a light trap at Rothamsted Experimental Station, Harpenden, form the basis of this stndy of the nocturnal activity of crane-flies of the sub-family Tipulinae (Diptera). The group was selected because of the economic importance of the larvae of a number of its species-notably Tipula paludosa, T. oleracea and Pales flavescens-and also because it was captured in numbers large enough to allow of statistical analysis of the influence of various climatic factors on its activity. The light trap, its construction, situation and method of working have been described by Williams (1935). It was kept in use continuously (with occasional accidental stoppages), for four years-March 1933 to February 1937. The captures of Tipulinae during the first two years have been analysed by Pinchin & Anderson (1936). The present study is a continuation and extension of their work. A further two years' captures have been analysed and the results for the whole four years summarized. It is important to remember that throughout this work the term " activity" is used in the sense of phototropic activity, i.e. the nunmber of insects caught in response to light-trapping.