Secondary forests are now prominent features in Neotropical landscapes, yet little is known about their conservation value for nonvolant mammalian communities. We performed a 20-month study using transect walks to survey the nonvolant mammal fauna in primary and secondary forests at La Selva Biological Station in the Caribbean lowlands of northeastern Costa Rica. We found that secondary forests can hold complex mammalian communities and no species exhibited habitat preferences between primary and secondary forests. With few exceptions, behavioral and group dynamic characteristics of mammal species were similar between forest types. Our research allows us to address methodological questions and assess the effectiveness of transect sampling. Monthly detection rates vary widely by species and likely reflect short-term changes in behavior rather than population fluctuations. Air temperature and rainfall are the factors most associated with monthly detection rates for various species. Small-scale areas of higher and lower use are evident for several species. Neither time of morning for surveys nor number of observers per trail affects detection rate. We report current abundances for the most commonly observed species and note generally lower densities in the Caribbean lowlands than elsewhere in the Neotropics. This research highlights that in less seasonal, evergreen forests climatic factors can still affect mammalian behavior. Overall, we find that secondary forests are of high conservation value and that transect walks are an effective methodology to sample many nonvolant mammal species, and we present recommendations on how to sample properly to conduct rigorous and long-term studies.