  We present mean angular diameters for two cepheid variables, alpha Ursae
Minoris and zeta Geminorum, determined with the Navy Prototype Optical
Interferometer (NPOI). We present linear radii for these cepheids and two
additional cepheids, delta Cephei and eta Aquilae, previously observed at the
NPOI. We find the limb-darkened angular diameter of alpha Ursae Minoris and of
zeta Geminorum to be 3.28 +/- 0.02 and 1.55 +/- 0.09 milliarcseconds
respectively. Using trigonometric parallaxes, we find the linear radii of alpha
Ursae Minoris, zeta Geminorum, delta Cephei and eta Aquilae to be 46 (+/- 3),
60 (+25, -14), 45 (+8, -6), and 69 (+28, -15) Solar radii respectively. We
compare the pulsation periods and linear radii of this sample of cepheids,
which range in period from three to 11 days, to theoretical and empirical
period-radius and period-radius-mass relations found in the literature. We find
that the observed diameter of alpha Ursae Minoris is in excellent agreement
with the predicted diameter as determined from both surface brightness
techniques and theory only if alpha Ursae Minoris is a first overtone pulsator.
