UPDATE 1-Mexico's Walmex Aug same-store sales up from year ago

* Monthly sales results exceed analysts forecast

* Company expects better H2 results

MEXICO CITY, Sept 10 (Reuters) - Mexico's biggest retailer Wal-Mart de Mexico said on Monday that sales at its Mexican stores open at least a year rose 4.1 percent in August from the same month a year earlier.

Results exceeded analysts ' expectations. According to a Reuters survey of five sector analysts, Walmex's same-store sales were expected to rise 2.8 percent last month.

Walmex, under investigation by U.S. and Mexican authorities over allegations that it bribed local officials to speed up store openings, ha s posted weaker performance against i ts p eers in recent months, p ro mpting it to reinforce its low-price marketing to attract custo mers.

During a Goldman Sachs conference last week, Walmex's Chief Executive Scot Rank t old analysts he expected the company to improve results in the second half of the year.

Walmex is the Mexican affiliate of Wal-Mart Stores Inc , the world's biggest retailer.

Walmex shares, down close to 7 percent year-to-date, ended off 0.78 percent at 35.57 pesos on Monday, before sales results were disclosed.