In a case closely watched by law enforcement officials and dog lovers alike, the court concluded that Aldo's alert signs during a routine traffic stop in Liberty County, Fla., had provided a sheriff's deputy with probable cause to conduct a search. The deputy didn't find the drugs Aldo was trained for, but did find materials used for making methamphetamine.

"Training records established Aldo's reliability in detecting drugs," Justice Elena Kagan wrote for all nine justices.

Although straightforward and relatively brief, the court's 11-page decision may have a broad reach for the many law enforcement agencies that rely on trained dogs. In accepting Aldo's training records as sufficient, the justices rejected a Florida Supreme Court ruling that would have required officials to produce a dog's field performance records -- including instances of false alerts, when drugs weren't found -- in order to demonstrate a dog's reliability when a search was challenged.

Kagan countered that the Florida Supreme Court's "strict evidentiary checklist" was too rigid in judging canine reliability, especially as "errors may abound" in a dog's field performance.

"The better measure of a dog's reliability thus comes away from the field, in controlled testing environments," Kagan wrote. "For that reason, evidence of a dog's satisfactory performance in a certification or training program can itself provide sufficient reason to trust his alert."

Underscoring the stakes, 24 states had sided with Florida law enforcement officials. The states -- including Pennsylvania, Texas, Washington and Idaho -- noted in a legal brief that "drug-detecting canines are one of the essential weapons in the states' arsenal to combat this illegal traffic."

A second drug-sniffing case out of Florida, involving a chocolate Labrador retriever named Franky and a 2006 house search conducted by the Miami-Dade Police Department, hasn't yet been decided, although it was argued before the Supreme Court on the same day.

Aldo, a German shepherd, had completed a 120-hour training course given by the Apopka, Fla., Police Department. He'd also received a one-year certification from Drugbeat, a private Missouri-based company that trains and certifies dogs for law enforcement. Once certified, Aldo and his handler, K-9 Officer William Wheetley, underwent regular training sessions.

Aldo and Wheetley were on patrol the afternoon of June 24, 2006, when the deputy sheriff pulled over a pickup near the town of Bristol, in Florida's panhandle. The pickup had an expired tag.

"The truck belonged to ... Clayton Harris," attorney Gregory Garre wrote in a brief for the state of Florida. "It was not going to be his day."

Harris seemed nervous, and the deputy saw an open beer can. He brought Aldo over and the dog got excited, and then Aldo sat down as an alert, next to the truck's front door. Under the driver's seat, Wheetley found 200 pseudoephedrine pills, which can be used to manufacture methamphetamine.

Harris' defense attorney, Tallahassee-based Assistant Public Defender Glen P. Gifford, noted that Aldo's original training had been with another officer and the dog's certification was out of date.

"Because a dog cannot be cross-examined like a police officer on the scene, whose observations often provide the basis for probable cause to search a vehicle, the state must introduce evidence concerning the dog's reliability," Florida Supreme Court Justice Barbara Pariente reasoned. "In cases involving dog sniffing for narcotics, it is particularly evident that the courts often accept the mythic dog with an almost superstitious faith."