In aquaculture, ponds with high loads of organic inputs, organic matter accumulates at the bottom over time. Uneaten feed, senescent phytoplankton and faeces are the principal sources of accumulated material, but quanti¢cations are scarce. The sedimented organic matter develops into a £occulent layer in which di¡erent processes transform the material into inorganic forms. A better understanding of factors in£uencing organic matter accumulation/decomposition in the sediment is needed to better understand and manage the dynamics of nitrogen in ¢sh ponds. In this study, the rate of mineralization of organic nitrogen and the nitrogen £ux between the sediment and the water column were measured. Organic matter accumulation in ¢sh ponds was quanti¢ed, and the data were used to construct, calibrate and validate a dynamic simulation model of organic matter deposition/decomposition in ¢sh ponds. The accumulating material consisted of dead phytoplankton, ¢sh faeces and uneaten feed. Through model calibration, the proportion of these materials in the total accumulated organic matter was determined. In the model, gross photosynthetic rate was estimated from an empirical relationship with feed input. After calibration, the model was validated using independent data. The model simulated well the concentrations of organic carbon and nitrogen in the sediments but it may be developed further, especially by considering the e¡ects of resuspension.