Photoelectrochemical (PEC) etching of c-plane GaN has been studied extensively because it is one of only a few available damage-free wet-etching techniques for this material system. Because of its nonpolar nature and low defect density, m-plane GaN may benefit from PEC etching even more than c-plane GaN has. With m-plane GaN, it is possible to achieve smooth, controllable etching, bandgap-selective top-down etching, and deep, anisotropic etching. We have investigated PEC etching of m-plane GaN in KOH and HNO3 and have found etch rates ranging from less than 10 nm/min to more than 1 mu m/min, with roughness that is crystallographic in nature but small in scale. Etch selectivity of 60:1 between In0.1Ga0.9N and GaN is observed using PEC etching with filtered light. Anisotropic etching to depths as great as 75 mu m was achieved, with the sidewall profile of the etch controlled by the direction of incident light.