UPDATE 1-Carrefour Brasil reports 5.5 pct rise in third-quarter sales

(Adds details on sales, comparison with GPA sales)

SAO PAULO, Oct 18 (Reuters) - Brazilian retailer Grupo Carrefour Brasil reported gross sales of 12.2 billion reais ($3.86 billion) excluding gasoline in the third quarter, up 5.5 percent from a year earlier, the company said in a securities filing on Wednesday.

Sales at stores open for at least 12 months, a gauge of demand known as same-store sales, rose 1.1 percent. Same-store sales at Atacadao, a retail format offering final consumers a wholesale experience, were up 1.6 percent, while same-store sales in more traditional formats were up 0.1 percent.

Food deflation weighed on revenue, Carrefour said. Food prices fell by 4.5 percent in the third quarter, according to government statistics agency IBGE.

Shares in Carrefour Brasil were up nearly 1 percent in midday trading on Wednesday, paring earlier gains of up to 2.7 percent.

The company's main competitor, GPA, reported on Monday that third-quarter net revenue grew 8.1 percent in yearly terms, while same-store sales rose 2.4 percent. Those figures beat Carrefour Brasil's sales performance but missed some analysts estimates, dragging GPA shares to two-week lows.

A bright spot for Carrefour Brasil was non-food sales, particularly electronics, in the group's traditional formats, which increased by double digits.

Gross sales increased 8 percent from a year ago in the first nine months of 2017, the company said, while same-store sales rose 3.6 percent.

($1 = 3.15 reais) (Reporting by Gram Slattery; Additional reporting by Jake Spring in Brasilia; Editing by Jane Merriman and Meredith Mazzilli)