The Flemish sheep breed is of the dairy type and currently considered as a rare breed in Flanders (Belgium). The original breeding objective of the Flemish sheep was wool production (18th century) but it was also known for its high litter size and good maternal capacity. In order to support conservation efforts, pedigree data (n=5,095) and litter size information (2,385 litters on 1,224 ewes) were analysed. In the period 2010-2015, there were on average 38 sires and 238 ewes used for breeding annually. The average inbreeding coefficient was estimated at 0.055 and the effective population size was 124 (based on increase of coancestry) and 55 (based on increase of inbreeding). Average prolificacy (2002-2015) attained 2.02±0.87 (mean ±stand. deviation) with up to five lambs born. Using a linear animal model and a Gibbs sampling algorithm, a relatively high heritability coefficient of 0.25 was obtained for prolificacy This result warranted a screening of the breed for major fecundity (Fec) genes known to affect litter size. Fifty-eight individuals of the breed (20 rams and 38 ewes) were screened at the 4 Fec gene loci for all known mutations in ovine BMP15, GDF9, BMPR1B and B4GALNT2. Four individuals (3 ewes and 1 ram) were identified as carriers of the FecXGr mutation in the BMP15 gene originally evidenced in the French Grivette breed and no other mutations were detected. This finding warrants: (1) a population wide screening of the breed for this mutation; and (2) an investigation into the effects of the mutation on prolificacy in this breed. This discovery also underlines the importance of conservation and investigation of endangered breeds because they may harbour valuable alleles for future use.