Hubway wants cyclists to "get spokey" by donning a Halloween costume and snapping a photograph near one of its bicycles or stations.
Cyclists can then upload the photo to Instagram, Twitter or Facebook with the hashtag #HappyHubwayHalloween or with the @Hubway tag.
More than 50 participants will be randomly selected from entries to receive prizes, including gift certificates to local restaurants Boloco and Bon Me. A Hubway annual membership is also up for grabs.
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Organizers asked on Hubway's website that participants put safety first.
"No in-motion selfies while riding, unbikeable costumes, or things dangling in spokes," the statement read.