Urban road dusts (URDs, n =10) were collected from arterial, residential, parking area in highway and highway roads in and around Tokyo, Japan, to characterize toxicity of size-fractions by the ostracod Heterocypris incongruens direct contact test. The URDs were collected with vacuum cleaner and highway sweeping vehicles, dried and size-fractionated before conducting toxicity test. The LC20 and LC50 of URDs varied (v/v) from 1.6 to 49%, and 3.8 to 67% respectively. Cluster analysis of URDs based on the concentration of heavy metal and PAHs standardized with the organic matter content was able to differentiate URDs into two groups, one group of higher toxicity and the other group of medium and lower toxicity. Mortality of ostracod decreased for some of the URDs when holding time of URD-water mixture was changed from 1 h to 24 h prior to the toxicity test. Fraction of fine particles was not always more toxic than the other fractions of coarse and medium particles. Site specific differences in toxicity of size-fractionated URDs indicated the complexity in defining URD toxicity as there could be co-existence of various non-targeted toxicants. (C) 2013 Elsevier B.V. All rights reserved.