ABSTRACT: This study evaluated the relationship between squid flesh transparency and muscle tissue microstructure. Squid mantle muscle was stored at 4 °C after being transported for 2 h by 4 different transportation methods used commonly in Japan (Group 1: live squid packed in ice-cold seawater; Group 2: live squid packed at 4 °C; Group 3: squid killed immediately after harvest and packed at 4 °C; Group 4: live squid packed in a fish tank containing seawater). Parameters of muscle tissue transparency were measured by an image analysis of digital images of squid muscle tissue. The mantle muscle tissue was observed under a transmission electron microscope to determine the postmortem structural changes at the cellular level. The ATP content of muscle tissue and rupture energy of squid flesh were also measured. As a result, the transparency of squid flesh and the ATP content of the muscles showed the same pattern of change in degree as time passed. The values of these parameters were highest in the group of squid killed immediately followed in order by those transported live, the refrigerated squid, and squid stored in ice-cold seawater. The mantle muscle tissue started to lose its transparency when the ATP in the muscle tissue started to decline. Disintegration of squid muscle tissue structure at the cellular level during storage under refrigeration for 24 h (4 °C) was observed in all methods of transportation. This suggested that destruction of the squid muscle tissue structure by autolysis is remarkably fast. The muscle tissue structure disintegrates due to decomposition of the muscle proteins, and muscle transparency is lost because the entire muscle develops a mixed coarse-minute structure.