Toyota Motor Sales U.S.A. Inc. reports sales of its core-brand Prius hybrid-electric car slid 15.4% in June, a rare decline for the vehicle that usually posts increases.

Bob Carter, group vice president and general manager-Toyota Div., blames a “shift in the market away from sedans.” He also tells a sales briefing today that low gas prices and slumping demand along the West Coast, a hotbed of Prius popularity, were contributing factors.

However, overall HEV sales are up slightly this year, according to Ward’s data.

In the year’s first six months, deliveries of hybrid cars and light trucks rose 2.1%, compared with like-2009.

The average price of a gallon of regular gasoline in the U.S. has hovered in the $2.70-range for weeks, AAA reports. This time last year, motorists were paying $2.63.

Passenger cars are fueling hybrid sales gains this year, up 7.8% through June, while hybrid light trucks have tumbled 20.1% in the period.

Despite the month’s dip, the Prius remains the best-selling HEV in the U.S. by a wide margin, with 66,039 units sold through June. Sales soared 41.2% last month, compared with May 2009.

While its sales are running 18.4% ahead of first-half 2009, the Ford Fusion HEV sales are growing faster. Fusion HEV deliveres spiked 85% through June to 10,008 units.

Of the 24 nameplates in the HEV market, just four others were in positive sales territory through June: the Chevy Silverado fullsize pickup and its GMC Sierra platform-mate, the Honda Insight and Nissan Altima.

Toyota also reports today its Lexus brand had one of its smallest monthly gains of the year, up 2.7%, as its June car sales fell – a trend mitigated by an uptick in light-truck deliveries.

Mark Templin, Lexus Div. group vice president, blames stop-sales of the LS and HS models for steering and fuel-leak issues, respectively, for part of the relatively poor June performance. He also points to high incentive and media spending by luxury competitors.

Templin says Lexus still is investigating engine-stalling problems related to contaminated materials of valve springs in 4.6L V-8 and 3.5L V-6 engines installed in older Lexus IS, GS and LS models. Once a remedy is found, Lexus will notify owners. Toyota is not advising owners to stop driving their cars.

Toyota Div.’s June car sales also were eclipsed by the growth in light trucks.

“We could have sold more light trucks during June if we had them,” Carter says.

Light-truck deliveries climbed 20.9% last month, as the Tundra fullsize pickup and new-generation 4Runner SUV saw sales hikes of 48.8% and 111%, respectively.

Carter says Toyota began building retail Tacoma compact trucks at its San Antonio, TX, plant this week, with higher production levels to kick into gear next week.

Carter is lukewarm on the U.S. auto industry, noting a recovery in the economy seems delayed. He says negative jobs and housing data likely contributed to consumers’ lack of interest in buying cars in June.

Still, Toyota sees a 2010 seasonally adjusted annual rate of 11 million units, on target with a January forecast.

Carter says Toyota is “doing fine in a market that continues to be very sluggish,” and is especially pleased with Midwest and Northeast regional sales.