Gamma USA Inc., a Miami curtain-wall contractor, has named James G. Mitchell as president of its Toronto-based Canadian operations. He succeeds Louis Fortin, who now leads engineering and production. Mitchell was executive vice president of Oldcastle Building Envelope, a supplier of architectural glass and aluminum glazing systems. Gamma also has named Mark I. Ospala as its New York City-based operations director. The firm ranks at No. 257 on ENR's Top 600 Specialty Contractors list, reporting $53.8 million in 2010 revenue.

Paul Hartwig has joined ACME Constructors Inc., an industrial construction firm that is based in St. Louis, Mo., as vice president of operations. He had been senior vice president of McCarthy Building Cos.

W.L. Butler Construction Inc., a Redwood City, Calif., building contractor, has elevated Daniel C. Warren and Brett Crail to vice president. Warren, based in Stockton, is director of operations. Crail, based in Irvine, is construction director. Butler ranks at No. 367 on ENR's list of the Top 400 Contractors, with $130 million in 2010 revenue.