Changes in the size of the area covered by individual cultured WI-38 cells as the cultures age have been studied by using a new microphotographic paper cutout technique. This method is nondestructive and noninstrusive and avoids a number of artifacts which can occur in the measurement of suspended cells. The measurements reveal that the decreased cell yield of late passage cultures reflects not only the appearance of a subpopulation of larger cells but also the failure of the cells to utilize all the growth surface available to them.