Hyphessobrycon innesi, the Neon Fish, suffers from a condition known as “Neon Fish Disease” that is invariably fatal. Study of a recent epizootic has shown that the causative agent of this disease is a species of Ichthyosporidium.

The symptoms of the disease and the distribution of the parasite Ichthyosporidium in the host tissues are described. The most heavily infected parts were the musculature and areolar tissue.

Ichthyosporidium is essentially histozoic and cytozoic. Nuclei of the host areolar tissue become hypertrophied, but those of the muscles are compressed or obliterated. Pressure effects prevent normal movements of the organs of the host.

The developmental stages and life history of Ichthyosporidium sp. in Hyphtssobrycon innesi are described and illustrated.

An attempt at treatment by increased water temperature and greater aeration removed superficially situated plasmodia but did not affect the deep-seated ones.

A brief summary is given of the distribution of other species of Ichthyosporidium and of their effects on freshwater and inshore and offshore marine fishes.