Golf balls having cores with negative hardness gradients are provided. The cores are made of polyalkenamer rubber compositions that may further include other rubbers such as, for example, polybutadiene, polyisoprene, ethylene propylene rubber, ethylene propylene diene rubber, and styrene-butadiene rubber. In one version, a solid, single core having an outer surface and center is provided, wherein the outer surface has a hardness substantially the same or lower than the hardness of the center to define a zero or negative hardness gradient. Dual-cores having inner and outer cores can be made, wherein the inner core has a negative hardness gradient and the outer core has a positive or negative hardness gradient. The rubber composition helps improve resiliency of the core and provides the ball with a comfortable and soft feel.