ABSTRACTRoot colonization as estimated by the line intersect method yields a quantitative value which is related to the extent of intercellular hyphal development and the proportion of the cortex occupied by cells with arbuscules. Hence, by knowing the amount of mycorrhizal colonization, we should be able to predict fungal biovolume provided the relationship (constant) between colonization and fungal biovolume is known. We have used data from morphometric cytological studies of mycorrhiza ofAllium cepaand a chitin assay forAgropyron smithiiroots to determine how colonization and fungus biovolume are related in order to derive the constant (Kf). We show that by using Kfand measuring root radius and colonized root length, we can calculate fungal biovolume and, hence, fungal biomass for an entire root system. We show that the regression model (P<0.0001) for the calculated mycorrhizal fungal biomass value (x) and the actual measured value as determined by the chitin method (y) are good predictors of biomass (y = 0.22 + 1.22x;R2= 0.81). Moreover, we show that when fungal biomass is expressed as percent of the root system, the relationship improves (y = 0.21 + 1.09x;R2= 0.89). This study indicates that it is possible to calculate ultraradical mycorrhizal fungal biomass from colonization data; a preliminary Kfvalue of 0.06 is proposed.