Cepheids are the primary distance indicators for extragalactic astronomy and therefore are of very high astrophysical interest .
Unfortunately , they are rare stars , situated very far from Earth .
Though they are supergiants , their typical angular diameter is only a few milliarcseconds , making them very challenging targets even for long-baseline interferometers .
We report observations that were obtained in the K ^ { \prime } band ( 2-2.3 \mu m ) , on the Cepheid \zeta  Geminorum with the FLUOR beam combiner , installed at the IOTA interferometer .
The mean uniform disk angular diameter was measured to be 1.64Â +0.14Â -0.16Â mas .
Pulsational variations are not detected at a significant statistical level , but future observations with longer baselines should allow a much better estimation of their amplitude .
The distance to \zeta  Gem is evaluated using Baade-Wesselink diameter determinations , giving a distance of 502 \pm  88 pc .