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Abstract The North Yellow Sea is a major aquaculture production area for the scallop Patinopecten yessoens is. In this study, the temporal and spatial variation of phycotoxins in scallops, phytoplankton, and their cysts were analyzed during a survey conducted from June 2011 to April 2012 around Zhangzi Island. The study area is a semi-enclosed epicontinental sea surrounded by the Shandong Peninsula, the Liaodong Peninsula and the Korean Peninsula. The three main results of the study were as follows: (1) The saxitoxin-group toxins, okadaic acid and analogues, and pectenotoxins were the major phycotoxin residues found in scallops; (2) Six kinds of toxic microalgae were identified, Protoperidinium spp., Gonyaulax spp., and Alexandrium spp. were the dominant taxa; Seven types of potential marine toxin-producing dinoflagellates, A. tamarense , A. catenella , Dinophysis fortii , G. catenatum , Gambierdiscus toxicus , Azadinium poporum , and Pseudo-nitzschia pungen were identified as the primary source of phycotoxins and were present at relatively high density from June to October; and (3) azaspiracids and domoic acid might be new potential sources of toxin pollution. This study represents the first assessment to phycotoxins around Zhangzi Island in the North Yellow Sea. Highlights • Toxin distribution occurrence in North Yellow Sea was investigated. • Saxitoxin, okadaic acid and analogues, and pectenotoxins were major phycotoxin in scallops. • Dominant phytoplankton and cysts were Protoperidinium spp. and Alexandrium spp. • Azaspiracids and domoic acid might be new potential sources of toxin pollution. [ABSTRACT FROM AUTHOR]