Bees of the genus Megalopta have gained attention as a result of their social nesting and nocturnal foraging. Seventeen nests of Meglaopta atra from the highlands of Chiriqui Province, Panama, were collected at the end of the dry season when brood rearing is expected to be at its peak. Most nests contained single females; within multifemale nests only one female possessed enlarged ovarioles, although some non-reproductive individuals were inseminated. In two of these nests reproductive individuals were clearly larger in body size than nestmates, but body size variation and macrocephaly were equivalent to those found in other Neotropical augochlorines. There was no evidence of a non-reproductive worker-like caste and multifemale nests did not appear to be more productive than solitary nests, which may represent pre-reproductive assemblages. Megalopta atra appears to be isolated by altitude from co-geners common in Panama, this is discussed in comparison with temperate halictine bees, in which environmental clines separate solitary from social populations. © 2008 Taylor & Francis.