Employing skin biopsies fixed in various ways, received in response to a questionnaire survey, sealpox was confirmed by light and electron microscopic examination in five captive California sea lions, Zalophus califomianus, one free-living harbor seal Phoca vitulina and one South American sea lion, Otaria byronia.

A nodular skin condition of unknown etiology is common among captive California sea lions, captive Stellar sea lions and captive elephant seals. A probable source of sealpox in aquaria and zoological gardens is infected animals captured in the wild.