Fillets of flounder (Hippoglossoides platessoides) and cod (Gadus morhua) produced in two commercial plants from uncut fish that had been frozen in blocks at sea, were free or nearly so of faecal coliforms and generally contained fewer indicator bacteria (all organisms in the coliform group; faecal coliforms) than fillets produced in the same plant from fish iced at sea. Coliforms were found in relatively small numbers on most fillets of thawed fish sampled on the conveyor leading from the mechanical skinner, on similarly prepared fillets sampled after trimming and portioning, and on fillets prepared from fish iced at sea and sampled immediately after skinning. The total plate count on the fillets of thawed fish at the point of trimming and portioning was small, suggesting that the fish frozen at sea carried a relatively low count of fish-spoiling organisms at time of freezing and thawing.The results show that in processing blocks of gutted or gutted and headed fish it is practical to thaw in an immersion thawer at a temperature of 13–20 C while changing the water only infrequently, say every 12 hr, and to employ thawing runs of about 24 hr duration. During this time as many as five tankfuls of fish may be thawed. It is practical also to hold the fish thawed under these conditions in chilled potable water or in crushed ice for 1 or 2 days prior to processing.It was shown that the sanitary aspects of the process can be assessed advantageously by observing the sanitary quality of the fillets, rather than that of the thawing water. The suitability of the method appeared to depend upon efficient washing of the thawed fish before it was put in buffer storage, or before it was filleted, or both. In one of the two plants sampled, large flumes were used and were satisfactory for delivering thawed, washed fish of high bacteriological and sanitary quality from buffer storage to the processing line. Here total plate and coliform counts either increased slightly or decreased slightly with prolonged use of the thawing water and of the boards used in cutting the fillets.