SUMMARYA numerical study has been undertaken to examine the behaviour of a gas liquid interface in a vertical cylindrical vessel subjected to a sinusoidal vertical motion. The computational method used is based on the simpliÿed marker-and-cell method and includes a continuum surface model for the incorporation of surface tension. The numerical results indicate that the surface tension has very little e ect on the period and amplitude of oscillations of the interfacial waves. The stability of the interfacial waves has been found to depend on the initial pressure pulse disturbance, and exponential growth of the interfacial wave has been observed in some cases. The in uence of the amplitude and frequency of the forcing oscillations has also been investigated. The results are in good agreement with available experimental and analytical solutions.