The endoplasmic reticulum (ER) of the sea urchin egg includes a system of cisternae, a cortical tubular network, and the nuclear envelope. In previous work (Terasaki and Jaffe, 1991, J. Cell. Biol. 114, 929-940) we showed that the long chain fluorescent lipophilic dye DiI spread from an intracellularly injected oil drop into all of these parts of the ER. In this previous study, we observed that in the first minute after fertilization, the ER cisternae became more finely partitioned; the large cisternae then reformed by 5-8 min after fertilization. To investigate whether these changes indicated a breaking up of the ER into discontinuous pieces, we have now examined the ability of DiI to spread in the ER at various times after fertilization. In eggs injected after fixation with aldehydes, we find that DiI spreads through the ER in unfertilized eggs and in eggs at 10 min postinsemination. However, DiI does not spread out of the oil drop in eggs fixed at 1 min postinsemination. We also find that in live eggs, the spreading of DiI is slower in eggs at 1-3 min postinsemination than in unfertilized eggs or in eggs later after fertilization. We interpret these observations to indicate that in the first few minutes after fertilization, the egg's ER is fragmented. This may be functionally significant for the early events of egg activation.