Extensive mud deposits are found off Cassino Beach, Brazil. The wave damping over the muddy bottom was studied using field measurements. By applying a technique of spectral analysis we showed that the wave attenuation occurred differently throughout the wave spectra. Field measurements revealed that the maximum wave energy dissipation took place over the deposit's depocenter and that lutocline height varied significantly in the order of days. The results indicated that short waves (from 3.75 to 6.25 s) underwent the greatest damping due to the interaction with fluid mud. An idealized 1-D model helped to explain the observations.@en