The conventional concept of OTF cannot be applied to sampled imaging systems which exhibit aliasing effects. This paper shows that the concept can, however, be extended to this class of systems if the condition of isoplanatism is redefined, and if only a limited range of spatial frequencies is considered. Parameters to quantify the amount of aliasing are also suggested, and these can be easily measured at the same time as the OTF, and used with the OTF to give a full description of the image transfer characteristics of aliased, sampled systems.