The structure of the accessory nidamental gland of the female squid, Loligo pealei, has been investigated using transmission and scanning electron microscopy. The accessory gland has many of the structural features of a secretory organ. The basic structural unit is a tubule composed of a single layer of epithelial cells containing ordered arrays of rough endoplasmic reticulum and a lumenal surface covered with microvilli, cilia, and structural specialization presumed to be involved in secretion. The lumen of each tubule is filled with a dense population of bacteria. During sexual maturation of the squid, the accessory gland changes in color from white to mottled red. The accessory gland of the sexually mature squid has a mixture of red, white, and yellow tubules; in each case, the color of the tubule is due to the bacterial population occupying the tubule. Since the red color of the gland is due to the pigmentation of the bacteria, the bacteria must be responsive to the sexual state of the host, possible through a change in the nature of the material secreted into the tubule lumen. The bacteria can be cultured easily. but in culture they fail to synthesize the red pigment.