Microplastic contamination is an increasing concern in terrestrial ecosystems, yet behavioral responses of soil-dwelling organisms remain less studied than conventional toxicity endpoints. This laboratory study assessed avoidance and mortality in three earthworm species, Eisenia fetida, Lumbricus terrestris, and Allolobophora chlorotica, and one soil-dwelling insect, Amara sp. (Carabidae), exposed to polyethylene microplastics at 0.10–0.30% (w/w). After 72 h, avoidance generally increased with polyethylene concentration, reaching 57.33% in E. fetida, 44.24% in L. terrestris, 51.90% in A. chlorotica, and 68.00% in Amara sp. at 0.30% polyethylene. Significant positive concentration effects were detected for E. fetida (odds ratio (OR) = 1.40, 95% CI: 1.10–1.79, p = 0.007) and Amara sp. (OR = 1.71, 95% CI: 1.33–2.19, p < 0.001), whereas trends in L. terrestris and A. chlorotica were not significant. Among the three taxa tested in the same soil matrix, polyethylene concentration significantly affected avoidance, while taxon and the taxon × concentration interaction were not significant. Mortality at 0.30% polyethylene was 12% in E. fetida, 26% in L. terrestris, and 36% in A. chlorotica, with none recorded in Amara sp. These findings support combining behavioral and mortality endpoints when assessing microplastic effects on soil fauna.