An approximate solution to the problem of an intense point‐source explosion at the interface separating two different fluids is described; in particular, it is applied to a nuclear explosion initiated on the surface of the ocean. Of special interest is the position of the deformed ocean surface as a function of time during the initial phase of the explosion. The equations of motion for an inviscid fluid with no heat conductivity are written in terms of the verticle angle and a similarity variable. The solution is valid for the early phase of the explosion, during which the counterpressure remains insignificant. It is found that the influence of the free surface on the vacuum interface propagated in the water is confined to a 45° sector adjacent to the original free surface. Below this, the vacuum surface retains very nearly its spherical form. This picture of the flow‐field form is seen to be in good qualitative agreement with the type of flow fields calculated by Brode and Bjork for a megaton burst on the surface of the earth.