Eggs of Ephoron album and E. shigae were studied by scanning electron microscopy. They showed similar structure of both polar cap and micropyles, but had different chorionic patterns. Micropyles were of the tagenoform type and, whereas in most mayflies thev are equatorial or subequatorial, in these eggs they were located close to the apical pole. The chorion was smooth in E. album and had a large mesh reticulation in E. shigae. The effect of water in causing the cap to swell and expand was tested by fixing eggs of E. album at different lime intervals after their mechanical deposition in water. The polar cap was a composite apparatus, consisting of finger-like projections with slight terminal enlargements, which arose from a common hood. The latter was connected via a thread bundle to the chorionic surface. In turn, the thread bundle arose from a chorionic collar located immediately underneath the hood. Each finger-like projection was composed of a large number of thin threads packed together, each ending in a small terminal knob. The function of the polar cap was discussed in relation to egg–water interaction.