A year earlier, 57 percent were deemed clean.
The annual report by the Straphangers Campaign looked at 22-hundred subway cars from 2008 to 09.
D can now stand for disappointing, as conditions on that line deteriorated, going from a rating of 80 percent clean to a dismal 38 percent.
The rating on the number 7 line has also dropped. In 2008, 84 percent of cars were clean. That number dropped to 63 percent in 2009.
Budget cuts are blamed with 52 fewer car cleaners now employed, according to Gene Russianoff of the Straphangers Campaign.
"It makes people think things are out of control, like why aren't the cars clean, and it doesn't help the reputation or image of the subways at all," he said.
So what constitutes dirty? The survey looked at the same criteria used by transit, dingy floors, sticky, smelly wet and dry spots, rolling bottles and food. Litter was not included.
It's not all doom and gloom, 29 percent of the cars on the "N" line were clean in 2008. That number went up to 63 percent last year.
NYC Transit conducts its own subway cleanliness survey. Its findings showed improvement in overall cleanliness for the second half of 2009.
The two surveys use similar but not identical methodology.