Theory is given for current‐carrier transport in nondegenerate semiconductors with anisotropic electron and hole mobilities and diffusivities. Two perpendicular principal directions are considered along each of which mobility of one carrier is maximum and mobility of the other minimum. This anisotropy might be obtained by elastic strain of a cubic semiconductor, or might occur without strain in certain semiconductors of noncubic structure. The ``photopiezoresistance effect'' in a slab with strongly absorbed radiation incident on one surface is analyzed in detail. Short‐circuit current and open‐circuit voltage between and electrodes are calculated, as well as efficiency of the transducer with an external load resistance. In the small‐signal range, the current, voltage, and efficiency are proportional to photon absorption rate. In the large‐signal range, with load matched for maximum power, similar proportionality holds for the current, while the voltage and efficiency have limiting large‐signal values. For maximum power there is an angle of optimum orientation of the principal directions relative to the surface of the slab. It is determined by the ratio of the maximum relative mobility variations for the respective carriers. Maximum attainable efficiency, the large‐signal value for pronounced anisotropy, is about 0.4 times thermal energy divided by the average energy required to produce one electron‐hole pair; the numerical factor is the root of a simple transcendental equation. This efficiency is about 0.4% for germanium at room temperature, and is consistent with an estimate from small‐signal theory with mobilities that are probably obtainable in strained material.