Human oocytes that failed to display signs of fertilization by 44 h after intracytoplasmic sperm injection (ICSI) were processed for electron microscopic analysis. All oocytes were arrested at metaphase II. The first polar body contained intact cortical granules and chromosome clumps, which were not surrounded by a nuclear envelope but still associated with microtubules. When a second globular body was present, it always showed the same ultrastructure, indicating that it had originated from fragmentation of the first polar body and not from the resumption of the second meiotic division. The most prominent organelles of the oocyte cytoplasm were the smooth endoplasmic reticulum and mitochondria. In the oocyte cortex, cortical granules were intact, with no signs of incipient or incomplete cortical reaction. Oocyte chromosomes were found in the oocyte periphery near the locality of the first polar body extrusion. They consisted of dense aggregates of chromatin associated with microtubules. The chromatin of the injected spermatozoon was demembranated and partially decondensed. In some cases, vesicular and tubular structures, apparently of oocyte origin, were associated with the periphery of the sperm chromatin mass but they never formed a continuous layer. These data suggest that fertilization failure after ICSI is basically a failure of oocyte activation.