This paper reports sound speed co, adiabatic acoustic nonlinear parameter B/A, and isothermal nonlinear parameter (B/A)' of 33 samples of excised human and animal fats and some simple mixtures, e.g., skim milk and powdered milk. Tissues were obtained from different parts of the body and the measurements were made in the temperature range of 20 degrees C to 37 degrees C. All the fats were highly nonlinear. Although there was a considerable overlap in the values of acoustic nonlinearity of different body fats, in general their values followed the order: mesenteric fat greater than subcutaneous fat greater than omentum fat greater than breast fat. Temperature coefficients of co and B/A, and the difference between adiabatic and isothermal nonlinearity, correlated highly with water/fat content of tissues. A reciprocal relationship between B/A and co, similar to that proposed for liquids, was observed for soft tissues but not for solutions of powdered milk in skim milk. No significant correlation was observed between acoustic measurements and the age of human subjects from which fat tissues were obtained.