We examined monthly and age-specific gametogenic development of the washboard mussel, Megalonaias nervosa, from April 1986 to March 1987 in navigation Pool 10 of the upper Mississippi River. We found M. nervosa to be a late tachytictic breeder. Female marsupia contained eggs or glochidia primarily from August (17°C) through October (9°C). Males were mature from July through October. Most females released their glochidia in October. Only one female was gravid in November (3°C). Most mussels were sexually mature at 8 years of age and then had an estimated average size of 68 mm (shell height). Only 8% of individuals ≤ 4 years of age showed any degree of reproductive development, while > 90% of age 5 and older individuals had recognizable reproductive material present. In host specificity studies, we verified three fish species as hosts for the glochidial stage. Green sunfish (Lepomis cyanellus), black bullhead (Ictalurus melas), and channel catfish (Ictalurus punctatus) produced juveniles after 26–28 days at 17°C. White suckers (Catastomus commersoni) and yellow perch (Perca flavescens) retained glochidia from 23 up to 26 days, but no juveniles were produced. Glochidia remained attached to common carp (Cyprinus carpio) and fathead minnows (Pimephales promelas) ≤ 3 days. Channel catfish were retested at 12°C and produced juveniles after 56 days.