The present study was conducted to determine the adverse effects of high incubation temperature on growth, development and genome stability of broiler chick embryo in ovo). One hundred twenty broiler eggs from Cobb Company, USA were weighted and divided into two groups. The first group was incubated at 37oC ± 0.5oC, and the second group was incubated at 41oC ± 0.5oC from 0 to 18th day. Starting on day 4th and every other day; three eggs from each group were examined following performed

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... nts as weight of eggs post incubation, embryo, yolk, and egg shell for measuring growth index. Blood smear was also prepared for counting heterophiles, and lymphocytes to determine H/L ratio. Micronucleus formation and presence of binucleated red blood cells were investigated as genome stability parameters, in 2000 cells. Significant reduction (P<0.01) in growth indices was observed in embryos grown at 41oC compared to those grown at 37oC ± 0.5oC. Reduction in H/L ratio was statistically significant (p≤0.01) in embryos of 2nd group comparing to 1st group embryos. Blood of embryo from heat stress group group (2) showed Red blood cells with micronuclei and binucleated cells while no such phenomenon could be seen in embryos from control group group (1). These results suggested that heat stress is influencing cell division at telophase and induces chromosomal damage. 88% of chicks from group (1) were hatched on day 21st; only 18% of chicks from group (2) were hatched lately on day 23rd, while the others were found dead. These results indicate that heat stress not only adversely affects growth and development of embryo stem cells but also induces genome instability which intern resulted in poultry production losses.