Structure and development of eosinophilic granules (EG1) in eosinophils from larva (ammocoetes) of far eastern brook lamprey Lethenteron reissneri were speculated. EG1 is stratified granules (two-layer) and consists of inner eosinophilic layer (L0) and chromophobic outer layer (L1). Three subtypes of EG1 are identified based on the optical artificial image (OAI) of inclusion structure (IS) in L0: EG1a, EG1b, and EG1c. The EG1a had no OAI (probably no IS). The EG1b and EG1c contain IS in L0. The OAI of both EG1b and EG1c were larger than IS. The former was round or oval chromophobic area (OAI-1), and the latter was expanded and rugged (three-dimensional) image (OAI-2) surrounded with OAI-1. EG1a was thought to be a prototype of EG1, which would develop into EG1b and then EG1c.