Microcebus simmonsi is the most robust of the eastern Madagascar mouse lemurs, with a head-body length of 12.8 cm, a tail length of just under 15 cm, a total length of around 27.5 cm, and an adult weight of 75–78 g (Louis et al., 2006a). The dorsal coat, head and forelimbs are dark reddish-brown to orange-brown, sometimes with a mid-dorsal stripe, while the ventral coat is grayish-white to white. There is a distinctive white patch on the rostrum, and hairs on the middle of the crown have black tips (Louis et al., 2006a).
As of 2009, this species had not been studied in the wild.
Northeastern Madagascar. This species is known only from the regions of Betampona, Zahamena and Tampolo (Louis et al., 2008).
The most recent IUCN Red List assessment (2008) classified M. simmonsi as Data Deficient (DD). It is currently known only from two strict nature reserves (Betampona and Zahamena) and Zahamena National Park. In 2009, there were four individuals of this species in captivity, all in the Tsimbazaza Zoo in Antananarivo, Madagascar (E. E. Louis Jr., pers. obs.).
Betampona is the best place to see M. simmonsi. It is a small reserve located about 40 km northwest of the major port city of Toamasina (= Tamatave), and is the site of a long-term lemur field research program. It can be reached by car, followed by hiking, and excursions can be arranged through tour operators in Antananarivo. This species can also readily be seen in Tampolo Classified Forest. Finding it at Zahamena is possible as well, but this area is much more difficult to reach.