Some of the subway's littlest riders are its biggest fare-beaters, a new study says.

Kids taller than 44 inches - the height limit for a free ride - made up 43% of fare-beaters observed by NYC Transit surveyors last year, an agency staff report says.

The "predominant mode of evasion is children over 44 inches ducking under turnstiles," the report says.

The surveyors noted that a subway surveillance camera even spotted a young boy enter without paying - and then open an emergency exit gate from the inside so his stroller-pushing mother could also ride free.