An adjustable fastener includes a male part and a female part, both of which are preferably made of soft pliable, sewable plastic. The female fastener part preferably has two or more spaced apart openings. Each having a broad portion and a narrow portion. The male fastener part has a base with an upstanding riser with a flange at the end of the riser. The female fastener part is made of a single thin wall which is sewn to the fabric of the garment. The base of the male part is also made of a single thin wall which is sewn to the fabric of the garment. The single wall construction of the female fastener part makes it more flexible, less easily seen, and not felt so much by the wearer. The female fastener part is provided with a bend between the openings so that it lies flat against the back when the male and female parts are coupled to the tightest position. The center of the riser of the male part is offset from the center of the flange. This allows the flange to be made larger and the narrow portion of each opening in the female part to be made smaller.