Iridovirus infections of the integument were associated with disease and mortality among hatchery-reared populations of juvenile pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus from the Missouri River. Virus-infected cells in the integument of fins and body were greatly enlarged, possessed pleomorphic and eccentric nuclei, and exhibited an amphophilic to eosinophilic staining of the cytoplasm in hematoxylin-and-eosin-stained sections. Virus particles found in the host cell cytoplasm were composed of an outer hexagonal capsid measuring 254 nm in diameter and surrounding a dense nucleoid. Despite numerous attempts, the virus could not be propagated on routine cell lines used in fish viral diagnostics or from established cell lines from white sturgeon Acipenser transmontanus, pallid sturgeon, or shovelnose sturgeon. Bath exposures of healthy juvenile pallid sturgeon to a crude extract or a 0.45-microm-filtered extract from the fins of infected fish resulted in transmission of the virus and mortality. At water temperatures of 15 degrees C, the first deaths occurred at approximately 1 month; mortality peaked between 50 and 60 d postexposure, after which surviving fish recovered. Presence of the virus was confirmed among dead and moribund pallid sturgeon by both histology and detection of viral DNA by polymerase chain reaction methods. Feeding of infected tissues and cohabitation with virus-infected shovelnose sturgeon also resulted in successful virus transmission to juvenile pallid sturgeon. Virus infections among experimentally exposed pallid sturgeon that recovered from clinical episodes persisted for at least 8.5 months, and these apparently healthy fish transmitted the virus and disease to juvenile pallid sturgeon by cohabitation. The newly described Missouri River sturgeon iridovirus (MRSIV) as found in pallid sturgeon and shovelnose sturgeon shares many properties with a group of iridoviruses associated with serious skin and gill infections in several species of sturgeon.