CnB and CnB2 clusters, n = 4-10, were studied at the B3LYP/6-311G** level of theory. When n = 5, the linear and cyclic structures are comparable in energy. For n greater than or equal to 6, the minimal-energy structures were found to be cyclic. Moreover, in contrast to all other reported heteroatom-carbon clusters, the lowest-energy linear structures were not those terminated by the boron atoms. These observations result from the electron-deficient nature of boron. The calculations predict that linear and cyclic forms will coexist for smaller cluster sizes, but that monocyclic forms will be the exclusive product as n increases. In their structural tendencies, the boron-carbon clusters are similar to pure carbon clusters.