* Denise Johnson headed Brazil's GM unit for eight months

* GM's South America head Ardila to be acting CEO

* GM's market share tumbled during Johnson's stint

The head of General Motors Co's (GM.N) Brazilian unit, Denise Johnson, has stepped down after just eight months on the job, the company said.

Johnson's resignation, announced on Tuesday, comes amid a decline in GM's market share in Brazil.

Jaime Ardila, chief executive of GM's South American unit, will take over as acting chief executive for the Brazilian unit, GM said.

Johnson, an engineer with experience in design and labor relations, is leaving GM after 22 years in search of new career opportunities, Ardila said in a statement on GM's website.

Under Johnson, GM's market share in Brazil slid to 17.7 percent in early February from 22 percent in January 2010, the newspaper Valor Economico reported on Wednesday. Just before Johnson was tapped for the Brazil CEO post, GM topped Volkswagen AG (VOWG.DE) as the country's second-largest carmaker, Valor said.

Italy's Fiat SpA (FIA.MI) has for years consistently led sales of family vehicles in Brazil. (Writing by Inae Riveras; Editing by Guillermo Parra-Bernal and John Wallace)