uneven exposure of the negative, but need more information. what kind of camera you using? Focal plane shutter? has it been serviced of late? Was this a fast exposure or slow one?

for example, if you have curtains on a horizontally running focal plane shutter not tracking evenly, and the two curtains are catching up to each other at times, that might give you this sort of banding effect, especially at a high shutter speed where the traveling slit is narrow and variations would show up more starkly.

this is highly unlikely to cause this sort of banding, however -- usually when that happens you get one end of the film unexposed, the other exposed.

you don't make it clear that this is nowhere else on the whole strip of film -- if it is, that might indicate a development problem -- lack of agitation, perhaps? Developer flowing over the film slowly instead of through proper agitation might explain the relatively even spacing of the banding because it flows through/around the sprocket holes.