Infectious bursal disease virus (IBDV) causes immunosuppression in chickens. We investigated the molecular changes in chicken embryo fibroblasts (CEF) adapted IBDV by genomic sequencing. IBDV were serially passaged in CEF and chickens were infected with the IBDV obtained after different numbers of passages in CEF. Chicken infections showed that 16th, 20th, and 21st passage viruses were pathogenic, while 26th and 36th passage viruses were non-pathogenic. Sequencing demonstrated that the initial changes during the serial passage comprised of a single-nucleotide deletion in the 3' non-coding region of segment B of the virus after 19th passage, followed by changes in the VP1 gene after the 20th passage of the virus and changes in VP2, VP5 after the 21st passage of the virus. These data suggested that the attenuation of very virulent IBDV was due to multigenic mutations and there are in vitro and in vivo competitive replications in IBDV quasispecies.