Disclosed are a process for the production of solid solution single crystals of KTiOPO.sub.4 wherein a part or all of at least one of the elements, K, Ti and P is substituted with one or more other elements, which comprises moving a grown part(s) of crystal(s) to outside of a melt for the crystal growth while maintaining contact of a growing part(s) of the crystal(s) with said melt to obtain the above solid solution single crystals, wherein the moving is carried out while maintaining the temperature of the melt substantially constant and maintaining said melt at a substantially constant composition at which the solid solution single crystal(s) with the desired composition is precipitated at the above maintained temperature and a solid solution single crystal of KTiOPO4 wherein a part or all of at least one of the elements, K, Ti and P is substituted with one or more other elements and its composition is substantially homogeneous within its cubic portions of a side length of 1 cm.