1. Three RNases were isolated from Physarum polycephalum and designated RNase Pp-1, RNase Pp-2 and RNase Pp-3. 2. They are thermostable. 3. The approximate molecular weights estimated by gel filtration of RNases Pp-1 and Pp-2 are 40, 000 and that of RNase Pp-3 is 10,000. 4. The optimum pH values of RNases Pp-1, Pp-2 and Pp-3 are pH6.7, pH 4.5 and pH5.5, respectively. 5. RNase Pp-1 is a guanyloribonuclease (EC 2.7.7.26). 6. RNase Pp-2 is a nonspecific RNase (EC 2.7.7.17). 7. RNase Pp-3 may be classified as a nonspecific RNase (EC 2.7.7.17), but it attacks purine nucleotide bonds preferentially.