Red cell numbers and haematocrit values were notable principally for their constancy during a 10-day experimental period following the exposure of goldfish acclimated to 20 C to an abrupt increase of 10 C in environmental temperature. Mean erythrocytic volume underwent a transient decrease while total blood iron (and presumably haemoglobin) and mean erythrocytic iron content fell slightly during the latter portion of the period of observation. Goldfish maintained at 5 C were typically characterized by two haemoglobin polymorphs, a third fraction being commonly found in animals acclimated at 12, 20, and 30 C. The observations are discussed in relation to the respiratory acclimation of this species to increases in environmental temperature.