SUMMARY The colony radial growth rate and germ tube specific growth rate of Absidia glauca decreased linearly with the logarithm of actidione, triphenyltin acetate or griseofulvin concentration. Colony growth rate was directly related to the mould's specific growth rate in submerged culture over a 200-fold range of actidione concentration. The radial growth rate of Afpergillus nidulans colonies was not influenced by inoculum concentration but colony diameter was directly related to the logarithm of the inoculum concentration over the lower range. INTRODUCTION Although colony diameter and radial growth rate are frequently used to measure fungal growth in bioassays and physiological investigations (Cochrane, I 958), few critical studies have been made to evaluate these growth parameters. The rate of growth of a mould can be defined most precisely in terms of its specific growth rate (a) or doubling time (td) in submerged culture. The specific growth rate of an organism may be calculated from the expression : a = ~n 2/td, where td is the doubling time. The reliability of colony radial growth rate as a para- meter of growth may be assessed by determining its relationship to the specific growth rate of the mould in submerged culture under various cultural conditions. Using this criterion the radial growth rate of Aspergillus nidulans colonies is directly related to the mould's specific growth rate over a wide temperature range (Trinci, 1969) and may thus be used to determine the optimum growth temperature for this, and presumably other, moulds. However, colony radial growth rate is not a suitable parameter to use to assess the relative growth rates of djfferent mould species (Trinci, 1969) or even mutants of the same species (Bainbridge & Trinci, 1969). Similarly colony extension cannot be used as a meaningful assessment of growth in nutritional studies (Trinci, Variation in colonyradial growth rate has frequently been used to measure the inhibi- tory effect of antifungal chemicals and the main object of this study was to determine if there was a correlation between the colony radial growth rate of a mould and its specific growth rate in submerged culture over a range of inhibitor concentration. In a previous study (Trinci, 1969) the value of germ tube specific growth rate as a para- meter of mould growth was investigated. We continue this study in the present work. 1969).