A parameter, σ₀=S₀α^-1 (gcm^-2), is defined. Where, S₀,(gcm^-3) is the initial substrate concentration and α, (cm^-1) is the specific area of the solid support. This parameter helps comparing the biomass yields, Y_X/S, of fungical superficial cultures either with a fixed S₀, value and variable α, or with fixed α, and variable S₀. A. niger cultures followed the logistic equation with maximal surface density ρ_AM (gcm^-2). Final average thickness, h, was measured by image analysis. The values of h, followed a saturation function of σ₀ (R² = 0.965) with extrapolated value h_MAX ≈ 0.4 cm. But the volumetric density, ρ_V = ρ_AMh^-1 was nearly constant ρ_V ≈ 0.046 ± 0.005 (gcm^-3). The regression Y_x/S^-1 = Y₀^-1 + σ₀ε^-1 (R² = 0.95) in the range, 5.9 mgcm^-2 < σ₀ < 62.8 mgcm^-2, indicated, Y₀ = 0.5 , as the maximal biomass yield, and ε = ρ_v h_MAX ≈ 0.02 gcm^-2, , as a measurement of the biosynthetic efficiency. This approach could be useful for the normalization and comparison of screening tests of fungal surface cultures with a variety of solid supports and culture conditions. For example, cultures with different S₀, values but, similar yield coefficient Y_X/S.