The presence of exogenous auxins such as indole-3-acetic acid (IAA), has been reported in organic materials such as vermicompost. The objectives of this work were to quantify the concentration of IAA in sand-based substrates with vermicompost (S-VC), to establish the relationship between the concentration of exogenous IAA and the amount of vermicompost (VC), as well as determine its influence on the development of tomato plants. S-VC substrates with different percentages based on volume (0, 20, 40 and 50% VC) placed in pots were used to transplant tomato seedlings. In each substrate, the concentration of IAA was determined at the beginning and at the end of a culture cycle, as well as the bacterial load. In addition, some performance components were measured in the plants. The results show that by increasing 1 kg of VC in a sand-based substrate, the concentration of exogenous IAA increases both at the beginning and at the end of the experiment (0.0470 and 0.0642 mg g-1, respectively); a result that, in turn, was related to the colony-forming units of gram-positive bacteria (Bacillus spp., mainly). Behavior that is reflected in tomato plants; for example, the root had its maximum development (371.33 cm-3) when the amount of VC in the substrate was 4.12 kg.