Single crystals of KMnPO4 grown in a KCl flux show ferroelastic domains. The structure was detd. on an optically controlled single domain crystal by x-ray diffraction methods. The cell is triclinic with dimensions a 5.4813(5), b 8.627(1), c 8.887(1) .ANG., a 87.73(1), b 89.10(1), g 88.01(1) Deg, VM = 419.6(1) .ANG.3 and Z = 4. The structure is closely related to the stuffed b-tridymite type. DTA expts. between room temp. and 1000 Deg and polarized light microscopy observations showed four phase transitions: at 165, 330, 353 and 707 Deg.