We studied the reproduction and shell size structure of the endangered dwarf wedgemussel (Alasmidonta heterodon) in the Mill River, Massachusetts, at sites of different mussel abundance, and assessed the dispersal capacity of its host fish, the tessellated darter (Etheostoma olmstedi). We quantified 4 phases of reproduction at 5 study sites of contrasting adult mussel abundance (3 low-abundance sites, 1 intermediate, 1 high), including gravidity, glochidial release, host infection rate, and juvenile recruitment. The ratio of gravid to nongravid individuals was higher at the high-abundance site, and comparatively lower at the intermediate- and low-abundance sites. Glochidial release, infection rates on host darters, and juvenile recruitment were directly proportional to mussel abundance. The highest glochidia density (0.12/m³) occurred at the high-abundance site. Mean infection rates during early May to late June ranged from 31% at the high-abundance site to 8% at the intermediate- and 0 to 2% at the 3 low-abundance sites. The high-abundance site showed highest level of juvenile recruitment. Low-abundance sites showed narrow ranges of size classes, probably indicating few year classes. Movement by marked darters was minimal during glochidial release, with 94% of marked darters remaining in locations where they were originally marked. Reproduction by A. heterodon in the Mill River depends on mussel patch density, and mussel dispersal by the host fish may be limited to the immediate vicinity of the infection site. Such a low dispersal capacity may lead to a patchy mussel distribution and may hinder colonization and recovery of this species.