Myeloid bodies (MBs) are distinct lamellar regions of the normally branched tubular smooth endoplasmic reticulum of retinal pigment epithelial (RPE) cells. These organelles are present in the retinas of many lower vertebrates. Previous investigations have reported a relationship between outer segment disk shedding and phago-cytosis by the RPE, and the formation of MBs. The current morphometric study was undertaken to establish if a temporal relationship existed between MB occurrence and the phagocytosis of shied photoreceptor outer segment tips in the chick retina. We report on the occurrence of phagosomes, MBs, and for the first time, MB precursors (templates) in the RPE over a 24-hr diurnal cycle.Phagosome numbers were observed to be highest within 2 hr following lights on, and again following lights off, while MB precursors were most prevalent at two time points, immediately prior to the times for rod and cone outer segment shedding at lights on and lights off respectively. Traditional MBs of the small (0.06 to 0.239μm2) and medium (0.24 to 0.99μm2) variety increased both in size and number from 01:00 hr of a 24 hr time period, with smaller and fewer MBs being present late in the dark part of the diurnal cycle, and larger and more numerous MBs in the later part of the light portion and into the early part of the dark portion of the cycle.This study has demonstrated a very different diurnal pattern of occurrence in both the size (organelle area as a percentage of total cell area) and number of MB precursors as compared to either small or medium-sized MBs, suggesting that MB precursors may represent a functionally, as well as morphologically distinct population of MBs.