The large cells recently suggested to be responsible for spoilage of packed cod, have been identified as Photobacterium phosphoreum. The spoilage activity of these cells, of Shewanella putrefaciens and of other microorganisms isolated form spoiled packed cod has been studied. Both qualitative and quantitative tests were used for characterization of the microbial spoilage activity. The importance of the different groups of microorganisms was evaluated by comparison of microbial spoilage activity determined in model substrates and in product experiments. The yield factor for production of trimethylamine (YTMA/CFU) and the cell concentration determined at the time of off-odour detection were used as quantitative measurements of microbial spoilage activity. On average cells of P. phosphoreum produced 30 times more TMA than cells of S. putrefaciens, YTMA/CFU of the two organisms were 10(-8.0) mg-N TMA/cfu and 10(-9.5) mg-N TMA/cfu, respectively. With these yield factors the level of TMA found in spoiled packed cod (30 mg-N TMA/100g) corresponds to about 10(7) cfu/g of P. phosphoreum and to 10(8)-10(9) cfu/g of S. putrefaciens. 10(7) cfu/g of P. phosphoreum were actually found in spoiled packed cod suggesting this organism could be responsible for spoilage. High cell concentrations of more than 10(8) cfu/g of S. putrefaciens were required for production of detectable off-odours and is was concluded that this organism is without importance for spoilage of packed cod.