WASHINGTON (AP) -- Only two of the 68 child safety seats tested for ease of use in the government's first-ever such rating score an "A" in all five categories.

Two models of the Graco Comfort Sport got an "A" in every category tested by the National Highway Traffic Safety Administration, which released the ratings Wednesday. No seat got below a "B" for an overall rating.

NHTSA gave 107 total grades for overall performance because it tested reversible seats twice - once forward-facing and once rear-facing. The agency gave 39 "A" grades and 68 "B" grades for overall ease.

NHTSA chief Dr. Jeffrey Runge said the ratings were positive but leave room for improvement.

"The new rating system is not only helpful to consumers, but also provides a strong market incentive to child seat manufacturers to make further improvements to their products," Runge said in a news release. Automakers often use NHTSA ratings in their advertising.

NHTSA said the 68 infant and booster seats it tested represent about 95 percent of the seats now on the market. Manufacturers tested included Cosco, Safety First, Britax and Evenflo.

Seats got separate grades on ease of assembly, clarity of labeling, clarity of instructions, ease of securing a child in the seat and whether the seat has features that make it easier to install in a vehicle.

Seven seats earned "C" grades -- the lowest grade given -- in two categories, but none of the seats got a "C" in more than two categories. The Safeline Sit and Stroll got "C" grades for its labels and instructions, while two Graco youth booster seats -- the Century Breverra Ascend and the Century Next Step -- got "C" grades for ease of installation and ease of securing the child respectively. Two Cosco seats -- the Regal Ride and the Ventura -- got "C" grades for labeling and instructions.

Representatives for Graco and Cosco could not be reached for comment Tuesday.

Congress required NHTSA to rate child seats in a 2000 auto safety act. NHTSA recommends that infants under 1 should be in rear-facing safety seats, while children between 20 and 40 pounds should be in forward-facing safety seats. Children who are over the weight limits for forward-facing seats but are still under 8 and under 4-foot-9 should be in booster seats.

Also Wednesday, the Insurance Institute for Highway Safety said that child seats don't always fit well into child seat latches mandated under a 2002 rule.

Under the rule, all new vehicles and child seats must have attachments designed to make them fit together like a key in a lock. The system, called LATCH, for Lower Anchors and Tethers for Children, is designed to make sure seats fit tightly in a vehicle.

In tests of 10 2003 model vehicles, the Insurance Institute found that the anchors were present but sometimes difficult to reach or secure. It said child seats were easiest to install in the Chevrolet TrailBlazer, the Dodge Grand Caravan and the Toyota RAV4 and most difficult to install in the Cadillac CTS and the Hyundai Santa Fe.

Susan Ferguson, the Insurance Institute's vice president for research, said parents should try fitting a child seat into their vehicle before buying one.

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