Costco Wholesale Corp. late Tuesday reported a surprising 3.2% decrease in first-quarter earnings, amid slowing growth in membership fees.

Membership revenue is a key source of profits for the warehouse retailer because it evens out fluctuations in sales and allows Costco COST, +0.05% to offer discounted products from large-screen TVs to organic string cheese, drawing more people to its stores.

In the latest period, membership fees rose 1.9% to $593 million, its slowest year-over-year growth in years. The percentage growth fell below the 6% increase reported in the year-ago quarter and the 3% rise expected by Nomura Securities International Inc.

Costco, in its news release, did not elaborate on its membership fees. The company will hold a conference call to discuss its results on Wednesday.

An expanded version of this report appears at WSJ.com.