Growth of the tall conidiophores of Aspergillus giganteus Wehmer in light was studied using time-lapse cinephotography. The conidiophores increase in height at an exponential rate until they are about 1 mm tall. This phase is followed by a period of progressively less rapid increase in growth rate until a constant rate is attained which persists for over 24 h. Conidiophore extension is restricted to a terminal zone c. 200 μ long. The height attained by conidiophores cultured in light is influenced by temperature and relative humidity; very tall conidiophores are only produced at saturated humidities. Growth of the conidiophores is terminated when vesicles are formed. The tips of conidiophores grown at light intensities of 20 lux oscillate about the vertical axis of growth; at 200 lux this ‘nutational’ movement is almost completely eliminated. The wall in the elongation zone stretches in a left-handed spiral manner similar to Castle's stage IV b sporangiophores of Phycomyces blakesleeanus. The conidiophores, which exhibit a strong phototropic response to unilateral illumination, are not geo-sensitive. When the light illuminating a culture is turned off, the conidiophores continue to grow for several hours at approximately the same rate as before but eventually there is a decline in the rate of extension and they finally stop growing, and form sporing heads. The growth of the conidiophores is discussed and comparisons are made with the growth of sporangiophores and vesiculate conidiophores of other species.