FORTUNE — In a note to clients Tuesday, William Blair’s Anil Doradla notes that although Apple’s (AAPL) iPhone is still America’s bestselling high-end smartphone, its sales momentum is “under pressure,” particularly at Verizon (VZ).

According to Doradla’s channel checks for the June quarter, the iPhone is no longer the No. 1 smartphone at Verizon’s retail outlets — the first time that’s happened since the carrier started selling it in February 2011.

Doradla attributes the iPhone’s fall from grace less to the usual slowdown in advance of a new model than to Verizon’s aggressive marketing of competing 4G devices, particularly Motorola’s Droid RAZR MAXX, which the carrier is selling for $299 with a 2-year contract. The same phone sells at Best Buy (BBY) for $799.99.

“Bottom line,” he writes, “while we acknowledge the increased competitive landscape from 4G devices (and the iPhone 4S is not 4G), we are not worried [about] Apple conceding its top position in North America any time soon. We believe the anticipated launch of a 4G device from Apple this fall will be an important catalyst for the company as consumers upgrade their phones, and we expect the company to once again maintain its top position at all operators (including Verizon Wireless).”

UPDATE: Canaccord’s Michael Walkley told AppleInsider today that he doesn’t believe any smartphone has overtaken the iPhone at Verizon, contradicting Doradla. It’s possible, however, that they are both correct. Walkley based his comment on checks conducted in April and May. Doradla’s survey covers June.

Below: Verizon’s bestsellers according to Doradla.