The following describes the data associated with Braga Goncalves, I., Mobley, K.B., Ahnesjš I., Sagebakken G., Jones A.G. and Kvarnemo C. 2015. Effects of mating order and male size on embryo survival in a pipefish. Published in Biological Journal of the Linnean Society. DOI: 10.1111/bij.12441 For further information, please contact Ines Braga Goncalves (inesvilargoncalves@gmail.com). The data in this file can be used to reproduce the primary analyses associated with the article. We kindly ask you to contact us prior to any novel analysis using these data, as well as to cite the data source and the publication noted above in any resultant products. The data set describes the body size measurements of male and female pipefish used in the study, the number of eggs brooded by the males, number of developing embryos after 18 days of brooding and average embryo length per brood at experimental termination. The column headers and their definitions are as follows: Male ID - code number of the experimental male. Male length - standard body length, from tip of snout to tip of tail peduncle, measured to the nearest mm. Male size - categorical variable where 1 = small males, 2 = large males. Mate number - categorical variable where 1 = first mate, 2 = second mate, for a given female. Female ID - code number of the experimental female. Female length - standard body length, from tip of snout to tip of tail peduncle, measured to the nearest mm. Female depth - measured across the thickest part of the trunk, recorded to the nearest 0.1mm. Eggs received - total number of eggs in the malesÕ brood pouch at experimental termination. Percent embryo mortality - percentage of unviable eggs in the brood pouch. This estimate includes unfertilized eggs and undeveloped and very underdeveloped embryos. Average embryo length - average embryo length for each brood, recorded to the nearest 0.01mm.