The lattice structure of ZnSe grown on GaAs by a low temperature low pressure organometallic chemical vapor deposition (CVD) process was studied using high resolution transmission electron microscopy. The defect structure of ZnSe epitaxial layers and of their interface with the GaAs substrate was directly imaged in cross section for GaAs substrate surfaces in the 〈100〉 and 〈111〉 orientations. It is shown that the ZnSe layers grow indeed epitaxially. The ZnSe layers grown on GaAs(100) contain a large density of faulted loops which are extrinsic in nature, whereas the prevailing defects in ZnSe layers grown on GaAs(111)B substrates are microtwins and stacking faults parallel to the film-substrate interface. A possible connection between the observed defect structure and the reported photoluminescence and electrical transport properties of ZnSe layers grown by organometallic CVD is also discussed. © 1983.