Abstract
The karyotype of the guinea pig, C. porcellus L. was investigated using chromosomal preparations from various tissues of 15 animals (nine females and six males). The diploid number is 2N = 64. Only four large autosomal pairs and the X chromosome are individually identifiable. One entire X chromosome and possibly the short arms of the second X in females show late DNA replication. A polymorphism involving the short arms of the longest pair of autosomes is present in this population and this polymorphism is shown to segregate in families.