The glochidial stage of the endangered freshwater mussel, Lampsilis higginsi, and several related species was studied to provide information on their early life histories. The glochidia of L. higginsi and the three species, L. radiata siliquoidea, L. ventricosa, and Ligumia recta, were compared using morphometrics and scanning electron microscopy (SEM). The glochidia of L. higginsi were morphometrically similar to those of the related species; however, they could be distinguished using SEM by the position of the hinge ligament and the dorsal ridge width. Fifteen species of fishes were tested for their suitability as hosts for the glochidia of L. higginsi. The following were found to produce at least one juvenile mussel: northern pike (Esox lucius), brook stickleback (Culea inconstans), bluegill (Lepomis macrochirus), green sunfish (L. cyanellus), largemouth bass (Micropterus salmoides), smallmouth bass (M. dolomieui), yellow perch (Perca flavescens), and walleye (Sitzostedion vitreum vitreum). A test of host quality using three members of the Family Centrarchidae ranked smallmouth bass highest with a transformation of 7.68%, followed by green sunfish (2.43%) and bluegill (0.00038%). Two propagation methods for juvenile Lampsilis mussels were tested. In vitro culture averaged 1.28 juveniles/plate, with a transformation of 1.05%. Transformation averaged 15-24 juveniles/fish on infected host fish. The pathogenesis associated with L. radiata siliquoidea on a suitable (walleye) and an unsuitable host (common carp) was compared using light and transmission electron microscopy. Encapsulation of glochidia on walleye gills was completed by 4-6 hr at 21°C. At 24-48 hr, the capsule was thin and compact. Fibrous tissue appeared in the capsule at 48 hr and increased in quantity to the end of the infection. Excystment occurred by thinning of the capsule aided by movement of the juvenile. Most of the glochidia attached to the common carp gills did not encapsulate. Partial capsular growth was evident in some, but the portions of the capsule distal to the bite consisted of necrotic cells and debris. A few complete capsules were found at 12-48 hr; however, all glochidia were sloughed by 60 hr. There was no evidence of leucocytosis; however, the number of heterophil type cells was greater in the capsular tissue of the common carp than in walleye.