The body is laterally compressed, particularly the rear third, where it flattens as it joins the caudal fin and forms an extended tail. The fire eel's base coloring is dark brown/grey, while the belly is generally a lighter shade of the same color. Several bright red lateral stripes and spots mark the body and vary in intensity depending on the age and condition of the individual. Usually, the markings are yellow/amber in juvenile fish, changing to a deep red in larger ones. Often the anal, pectoral, and dorsal fins have a red edging.