NEW YORK — Retailers said sales ticked higher in May as warmer weather brought more shoppers into stores. Memorial Day promotions contributed to the improvement.

Retailers said sales ticked higher in May as warmer weather brought more shoppers into stores. Memorial Day promotions contributed to the improvement.

Average sales growth at nine U.S. retailers, including Costco Wholesale Corp. and Rite Aid Corp., beat expectations in May, according to the Thomson Reuters same-store sales index. Sales at established stores on average rose 4.3%, exceeding the 3.9% analysts had been expecting for May.

Thomson Reuters reported that mall traffic improved late in the month. Similarly, many retailers posting quarterly financial results in recent weeks noted that sales trends began accelerating in April and were sustained in May.

Many retailers no longer report monthly metrics, making it hard to monitor short-term fluctuations in the sector.

Costco, one of the retailers still reporting monthly sales data, said sales climbed 6% in May, excluding gasoline. The results topped analysts’ expectations that the Issaquah, Wash.-based company would post a 5.3% sales increase for the month.

Among the best-performing categories at Costco last month were fresh food, small appliances, housewares, apparel, automotive, and lawn and garden. Costco said it had weaker sales in the hardware category and in consumer electronics. The company also said May sales were hurt by foreign currency changes.

Rite Aid’s same-store sales for the five weeks ended May 31 rose 3.5% from a year earlier. The drugstore operator said front-end same-store sales were up 0.5% in the period.

Pharmacy same-store sales were up 5%, a gain curbed by what the Camp Hill, Pa.-based company said was a negative impact of 156 basis points from the introduction of new generic drugs.

Walgreen Co. said May sales rose 0.6% to nearly $6.6 billion. Sales at stores open a least a year rose 4.4% in the period. Front-end comparable-store sales were up 2.6%, despite an 0.5% decline in customer traffic. Walgreens said basket size increased in the period by 3.1%.

The Deerfield, Ill.-based company said pharmacy sales in May rose 7.9%, with the gain negatively affected by calendar-day shifts as well as generic drug introductions. Pharmacy sales accounted for 64.2% of Walgreens’ total sales in May.

Discount retailer Fred’s Inc.’s sales slid 0.4%. Sales strengthened in the last week of the month after the launch of a new marketing and branding program designed to increase store traffic and raise customer awareness of category diversity.