Abstract: A swim fin having open instep and heel portions. The swimmer's foot is retained in the fin by means of an instep strap configured to pass over the instep of the foot and a heel strap that surrounds the heel of the foot. The instep strap passes through a pair of longitudinal slots in the planar foot portion of the fin and up and over the foot such that the ends of the instep strap are secured at the instep. Preferably, to facilitate adjustments, the ends of the instep strap overlap one another and are secured with hook and loop fasteners. A locking strap overlays the instep strap at the instep of the foot and further secures the fin to the foot. The instep strap conforms to the instep of the foot and lends itself to enclasping the arch of the swimmer's foot. Preferably, the heel strap is integrally formed from the instep strap and the ends of which overlap one another are secured with hook and loop fasteners. A heel locking strap overlays the heel strap at the ankle and further secures the fin to the foot.