Abstract: Additions to the Costa Rican ant fauna are reported. Camponotus senex (F. Smith, 1858) is redefined and C. senex textor Forel, 1899 is raised to species. The natural history of these two species is clarified, with C. senex nesting in dead branches and C. textor constructing leaf nests sewn together with larval silk. Earlier literature accounts of silk use by C. senex are misidentifications of C. textor. Pachycondyla obsoleta (Menozzi, 1931) is synonymized under Cryptopone gilva (Roger, 1863). Nesomyrmex tristani (Emery, 1896) is synonymized under N. asper (Mayr, 1887) and N. antoniensis (Forel, 1912) is removed from synonymy and raised to species. The following new species are described: Adelomyrmex brenesi sp.n., Lenomyrmex colwelli sp.n., Nesomyrmex vargasi sp.n., Pyramica augustandrewi sp.n., P. cascanteae sp.n., P. erikae sp.n., P. oconitrilloi sp.n., P. paniaguae sp.n.