Petroleum based-diesel fuel (PD) synthesized from crude oil contains numerous toxic compounds, which pose a serious threat to the environment. Biodiesel (BD), synthesized primarily from soybean oil, contains fewer of these toxic compounds and hence, may be more environmentally friendly than petroleum diesel. However, there is little scientific data to confirm this. The objective of this study was to compare the relative toxicity of BD to PD and PD-BD blends in a simulated spill on soil by examining plant growth during a 28-day greenhouse experiment. A loamy topsoil was contaminated with either PD, a blend of 98% PD and 2% BD (B2), a blend of 80% PD and 20% BD (B20) or 100% BD (B100) at rates of 0 (control), 10, or 50 g diesel kg^-1 soil, which was placed in one-liter pots in a greenhouse and seeded with either 10 seeds of Glycine max (soybean) or Zea mays (corn). The experimental design was completely randomized with five replications per treatment. Plant emergence and height were measured on days 7, 14, 21, and 28. Plants were harvested on day 28 and roots and shoots were separated at the crown and dried in an oven at 60 °C to measure biomass. Populations of bacteria and fungi were enumerated on days 7, 14, and 28 using serial-dilution and plate-counting techniques. Diesel was extracted from soil on days 7, 14, and 28 and measured using gas chromatography. Results show both soybean and corn plants tended to emerge faster, and produce more biomass in treatments contaminated with B100 than with PD, B2 and B20. No significant differences in bacterial and fungal populations were observed between diesel treatments. Disappearance of B100 from soil was significantly faster than the other diesel treatments. Results indicate that B100 may be more environmentally friendly when accidentally spilled than PD.