The reproductive biology of fishes impact many other components of their life history, and can influence their vulnerability to fisheries, therefore for more informed management, a good understanding is essential. For lumpfish (Cyclopterus lumpus), a semi-pelagic species found across the north-Atlantic and targeted by fishers for their roe, comprehensive knowledge of this aspect of their life history is still lacking. Through a combination of regular sampling from scientific surveys and fisheries and modern methodology in fish reproductive biology, we investigated the ovary development of lumpfish throughout vitel- logenesis. The results showed that ovaries of lumpfish had a wide range of oocyte sizes and that lumpfish are a determinate, batch spawner with ovary development taking at least 8 months. They spawn a maximum of two batches per season with a similar number of eggs in each batch. Unusually for a determinate batch spawner, the two batches were easily distinguished within the ovary prior to ovulation. Average egg size ranged from between 2050 and 2500 µm, with larger fish having larger eggs, and the egg diameter of the second batch being on average 1.6% smaller than the first. Lumpfish were documented as spawning over a 4-month period, but it is likely that spawning occurs over a greater period. A macroscopic and oocyte size frequency distribution (OSFD) scale for lumpfish is presented which can be used for future studies of lumpfish.