Blood parameters for 31 species of marine, estuarine and fresh water fishes from Venezuela were correlated with habitat, activity level and type of respiration. Hemoglobin polymorphism was found in Astyanax bimaculatus, Plecostomus vatwata and Thalassophryne maculosa, but no relationship was found between presence of polymorphism and activity level, type of respiration, or instability of the environment (thermolabile or thermostabile). The number of hemoglobin was unrelated to activity level, type of respiration and habitat (marine, estuarine or freshwater), but hemoglobin concentration, hematocrit and number of erythrocytes were higher in highly active species than in species having low activity levels. High activity level was correlated with branchial respiration and low activity level with aero-branchial respiration. Fishes having hemoglobin with high oxygen affinity had higher values of pH and Root effect. Freshwater species showed a greater oxygen affinity than estuarine or marine species, and Root effect was higher in marine species.