Police asked straphangers Friday to be on the lookout for a mugger who has robbed at least 11 subway riders in the past few months.

Police released a surveillance photo and a sketch of the suspect, seen above, who has struck four times since the beginning of June.

They say he approaches riders and either pulls out a knife or indicates he has a weapon. Then he demands their belongings, bank cards and PIN numbers.

Detectives have also released a sketch. They describe the robber as six-feet-one-inch to six-feet-two-inches with a medium build.

He has black hair and appears to be 20 to 25 years old, and was often seen wearing a black baseball cap.

Police also asked for help Friday in tracking down a would-be bank robber in the Bronx.

Authorities said a woman, seen right, walked into the Citibank on 198th Street in Bedford Park last Saturday.

Police say she passed a note asking for money from the clerk, but escaped before taking any.

The suspect is described to be in her 30s, about five-foot-four-inches and 115 pounds, wearing a dark hooded sweatshirt, black jeans and black suede boots.

Anyone with information on either case should contact the Crime Stoppers hotline at 1-800-577-TIPS, or text CRIMES and then enter TIP577, or visit www.nypdcrimestoppers.com.