Vibrations of square plates made of composite materials immersed in fluid have been studied in the present work. Water has been chosen as the fluid medium and the finite element method is used for the analysis for both fluid and plate. Square plates having a single lamina and unidirectional fibre are considered. The eigenvalues, eigenvectors and the corresponding mode shapes of the plates in vacuo and in fluid have been determined. For different boundary conditions of the plate, the influence of fibre orientation and the influence of plate material to fluid densities towards the plate frequencies have been studied and the results are shown in the form of tables and graphs. © 1990.