The remineralization of phytoplankton-bound phosphorus subsequent to nonpredatory phytoplankton mortality represents a significant source of algal-available phosphorous in many lakes. A unique experimental apparatus (A Dual Culture Diffusion apparatus) was used to measure the rate and extent of the process and to elucidate some of the governing factors. It was demonstrated that the process is strongly influenced by heterotrophic decomposer activity, because phosphorus regeneration rates were less than 0.01/d for cultures not inoculated with a decomposer community, while they were two to five times higher for decomposer-inoculated cultures. In addition to the character and activity of the microbial decomposer community, the phytoplankton cell phosphorus content was shown to be a significant factor in the rate of phosphorus regeneration for a given cell decay rate. Cell phosphorus above the minimum cell quota appeared to be released in an available form quite rapidly upon algal death and lysis.