The original McMaster slide (Gordon and Whitlock, 1939) has been extensively used for investigations involving dilution counts on helminth ova in faeces. In view of the simplicity inherent in its design and use, attempts have been made to utilize it for obtaining estimates of the viable egg contents of cysts of Heierodera rostochiensis, the use of a modified McMaster slide for this purpose being described by Fenwick (1942). Fenwick's slide differed from the original in that the counting area was marked on the floor of the counting chamber instead of its roof; this modification resulted from the use of water as a diluent instead of semi-saturated brine as used in faecal examinations. The capacity of the original McMaster slide (0·15 ml.) was, however, found to be too small to give a reasonably large count from a batch of 100 cysts unless the degree of dilution was so small that efficient agitation before the withdrawal of a sample for counting was difficult.