Biodegradable plastics have been developed to eliminate the progressive accumulation and ever-growing threat posed by conventional fossil fuel-derived plastics. The impact of these bioplastics, particularly in an agricultural context (e.g. biopolymer mulch films), however, remains poorly understood. In this study, we compared the biotoxicity of biodegradable (polylactic acid, PLA; polypropylene carbonate, PPC) and non-degradable (polyethylene, PE) microplastics using a series of standardized bioassays using the earthworm Eisenia fetida. The responses studied included: avoidance behavior, mortality, biomass, and reproduction responses. We incubated earthworms in artificial soils amended with different concentrations of microplastic (0, 0.125, 1.25, 12.5, 125, 250, and 500 g kg−1) under laboratory conditions. This wide range allowed linear regression modeling and estimation of microplastic effect thresholds. Our results showed that microplastic concentration rather than plastic type was more important in regulating earthworm responses to soil contamination. The critical threshold for microplastic contamination was 40 g kg−1, after which earthworms exhibit microplastic avoidance behavior. A significant reduction (EC10) in number of cocoons and juvenile earthworms occurred at a concentration of 53 g kg−1 and 97 g kg−1, respectively; while no significant effect was found for survival of earthworm until levels of 500 g kg−1. Overall, the two biodegradable materials (PLA and PPC), appeared to be no more biofriendly than PE. Based on reported levels of plastic contamination in soil of up to 67 g kg−1, we conclude that microplastics are now starting to pose a threat to earthworm population. To better evaluate the risk posed by biodegradable and nondegradable plastics, further mechanistic studies on how microplastics affect earthworm behavior and the potential long-term impacts of this on soil functioning are required.