OBSERVATIONS ON THE BIOLOGY OF MACROBRACHIUM AMERICANUM FROM A 0.61-HECTARE POND ON THE PACIFIC SLOPE OF CENTRAL PANAMA WERE MADE FROM 1970 TO 1973. SPECIMENS WERE TRANSPORTED TO AUBURN UNIVERSITY IN 1970, AND THEIR GROWTH RATE AND COLD TOLERANCE WERE OBSERVED IN PONDS. FECUNDITY AND EARLY LIFE OF LARVAE WERE STUDIED IN THE LABORATORY. SPAWNING IN THE PANAMA POND OCCURRED FROM JUNE TO NOVEMBER, WITH A PEAK IN SEPTEMBER WHEN 55% OF ALL CAPTURED FEMALES HAD EGGS. METHODS FOR DETERMINING THE SEX OF IMMATURE PRAWNS WERE DEVISED. LENGTH-WEIGHT RELATIONSHIPS FOR THE POPULATION WERE DETERMINED. FEEDING EXPERIMENTS IN AQUARIA AND THE WIDE VARIETY OF PLANT AND ANIMAL MATERIALS USED SUCCESSFULLY AS BAIT, INDICATED THE OMNIVOROUS FOOD HAVITS OF MACROBRACHIUM AMERICANUM. RECAPTURES OF IDENTIFIABLE SPECIMENS SUGGESTED TERRITORIALITY. THE IMPORTANCE OF SPACE TO INDIVIDUAL ADULT SHRIMP INDICATED FURTHER BY THE POSITIVE CORRELATION OF NUMBER OF SPECIMENS CAPTURED WITH AREA OF TRAP BOTTOM. (SIC).