MEXICO CITY (Reuters) - Mexican conglomerate Grupo Carso posted a 33.7 percent decline in first-quarter net profit, hurt by declining operating margins.

Carso (GCARSOA1.MX: Quote, Profile, Research), controlled by Mexican billionaire Carlos Slim, said on Friday it earned 1.321 billion pesos ($124 million) in the January-March period.

Carso operates retail, infrastructure and industrial businesses.

"The reduction in operating profits is explained by lower margins in (Carso's) three divisions," the company said.

Sales at Carso were 18 billion pesos in the first quarter, up 5.2 percent over the year-ago period.

Operating profits fell 12.5 percent to 2.04 billion pesos.

(Reporting by Noel Randewich)