A colorimetric method of estimating vesicular-arbuscular mycorrhizal infection was developed and tested using onions inoculated with Glomus fasciculatus. Absorbances at 400 nanometers of water extracts from a series of mixtures of yellow (mycorrhizal) and white (nonmycorrhizal) roots had significant correlation with the percent yellow roots by weight in the mixtures. The percentage of mycorrhizal infection determined by a chitin assay or by a staining method was significantly correlated with the percentage of yellow roots by weight. The percentage of mycorrhizal infection determined by the colorimetric method was also significantly correlated with root/shoot ratios. The colorimetric method is offered as an alternative to the more time consuming quantitative methods of estimating the extent of vesicular-arbuscular mycorrhizal infection now in use.