The effects of some environmental and genetic factors on the isoelectric focusing (IEF) patterns of muscle proteins from a fish Lebistes reticulatus (Cyprinodonts) were investigated. Of two diets applied the restricted one resulted in a loss of bands in the IEF patterns of the treated fishes muscle homogenate. Instead immunization gave an increase in both number and intensity of IEF bands in (4.0-6.0; 7.0-7.9) pH ranges which are typical of immunoglobulins. Lastly IEF patterns from random and assortative mating animals were compared, resulting in loss of IEF-bands in the inbred lines. Results showed that IEF of muscle proteins could be an excellent method to evaluate the response by fish phenotypes to environmental or genetic factors.