As Moseley (1879) noted, it may be imprudent to assume there must always be a ‘reason’ for observed displays of bioluminescence. However, knowledge of the colour of the light emitted may be useful for examining some of the theories about the function of marine bioluminescence. Certainly the colour of the light is important in predicting the rate of its attenuation in sea water, and the ease of perception of the light by the organism itself, or by potential predators, prey, or mates. In addition, the colour of the light may indicate something about the compounds responsible for producing the light. The present paper presents some bioluminescent emission spectra of freshly caught organisms from the Sargasso Sea.