Chronological changes of the chromosomes upon feritilization of the giant freshwater prawn,Macrobrachium rosenbergii(de Man), were followed. Egg chromosomes, within 2 min after spawning, were at the second meiotic metaphase whereas sperm nucleus retained a cupped shape of the base region. The egg chromosomes progressed to the second meiotic anaphase by 10–15 min and completed maturation by 20 min after spawning. Both female and male pronuclei were clearly differentiable at 1 hr after spawning. Female pronucleus was larger than the male pronucleus. But both pronuclei enlarged to an equal size when they were in proximity by 3 hr after spawning. Pronuclear fusion took place by 4 hr after spawning. The fused nucleus advanced into the first mitotic division rapidly so that the first mitotic metaphase or anaphase was observed by 5 hr after spawning. The first and the second nuclear divisions were completed by 6 and 8 hr after spawning, respectively. While the first and the second cytokineses occurred simultaneously at 9 hr after spawning resulting in a 4-cell embryo. Observations of chronology of the chromosomal events upon fertilization provide informations concerning the crucial times for further chromosome number manipulation.