This is a medium-sized guinea pig with an adult length of about and a weight of . The tail, at around 2.4 mm, is almost non-existent. The dorsal fur is dark olive-brown mixed with brown and black, and the underparts are a pale grey or yellowish-grey. Its
karyotype has
2n = 64 and a reportedly variable
FN of 116 or 128 for
C. a. aperea and 128 for
C. a. pamparum. ==Distribution==