The effect of the composition of a mixture containing, oat straw (OS), oat bran (OB) and copra cake (CC), on the mycelial growth of Pleurotus ostreatus was studied using mixture and response surface methodologies. The applied constraints to the mixtures were. moisture content higher than 70%, C/N ratio less than 30 and total mixture cost less than 2% of the retail cost of fruiting bodies of P. ostreatus. The maximum observed value of apical growth rate (0.50 +/- 0.02 cm day(-1)) was obtained using 0.633, 0.284 and 0.083 (g g(-1) mixture, dry basis) for OS, CC and OB mass fractions, respectively. Under these conditions the C/N ratio was 22.4-23.2. Loss of dry matter decreased from 16.9 to 8.5% as the OS fraction (lignin and cellulose source) was increased from 0.55 to 0.80 (g g(-1) mixture, dry basis). The utilisation of mixture and response surface methodologies was an useful approach to evaluate the relationship between substrate composition and mycelial development of P. ostreatus.