We conducted a field survey in Japanese streams and rivers to generate useful information about whether the Japanese water quality standard for zinc (Zn) in the freshwater environment (30 mu g/L) is over- or underprotective of the populations and communities of lotic benthic macroinvertebrates. Many macroinvertebrate populations were reduced and community characteristics were altered at two polluted sites with Zn concentrations above 1,000 mu g/L as compared with control sites with Zn concentrations below 30 mu g/L. In contrast, at least one site among three polluted sites with Zn concentrations of 80-126 mu g/L experienced noncritical effects (i.e., no significant difference was observed between the sample mean at a polluted site and that at the control sites, and the sample mean was within the minimum-maximum range of those at the control sites) for most of the ten community metrics for diversity and abundance. In addition, the abundances of 95% of the dominant taxa were not always significantly lower at more than one polluted site (Zn concentrations, a parts per thousand yen80 mu g/L). Our results suggest that Zn concentrations below approximately 100 mu g/L are likely to protect most aspects of richness and abundance of the riverine macroinvertebrate communities in the study areas. This study shows a case that the Japanese Zn standard is likely overprotective of the macroinvertebrate populations and communities.