Urban ecosystems can support high levels of plant and animal biodiversity. Urban spontaneous vegetation (USV) sites in Halifax, Canada consist of relatively unproductive vacant lots on poor soil, contain roughly 50% native plant species, but support higher invertebrate diversity than nearby urban lawn and forest habitats. We determined environmental predictors of invertebrate diversity in USV habitats. We hypothesized that site-level invertebrate diversity would increase with plant species beta diversity and spatial environmental heterogeneity. We sampled 11 sites, with plant communities, soils, and invertebrates (using sweep netting and pitfall traps) quantified in 1m x 1m plots. Site-level pitfall trap morphospecies richness ranged from 24 to 56 and was dominated by insects but also included spiders, mites and snails. Site-level sweep net richness (22-79 morphospecies) consisted exclusively of insects. Site-level plant species richness ranged from 40-158 species (mean plot richness: 12-22). Pitfall trap morphospecies richness was positively correlated with mean plot-level plant richness, soil depth heterogeneity and site area. The only signficant predictor of sweep net richness was the individual density of insects caught, which was positively correlated with mean plot plant richness (alpha diversity) and site area. Neither productivity nor plant beta diversity indicators predicted invertebrate species richness in this study.