Technical Abstract: The objective of this study was the description of changes in genetic diversity in a colony of dog guides since its founding. Two breeds of dog, German Shepherds (GS) and Labrador Retrievers (LR), were evaluated. Data were pedigrees of 4,699 GS and 3,573 LR dogs bred for use as guides by The Seeing Eye, Inc., Morristown, NJ. Rapid increases in average pairwise numerator relationships occurred in both breeds, although the average was approximately one-third higher in the GS population than in the LR population. A similar trend was observed for average inbreeding. The rate of increase in inbreeding has slowed in recent generations. In the current generation, relationship and inbreeding for all animals averaged 25.3% and 26.2% in GS and 15.5% and 22.0% in LR, respectively. Effective founder numbers initially decreased in GS until generation 3, and then increased steadily. Effective founder number in LR constantly increased. There was a constant increase in effective founder number in LR. A similar pattern was seen for effective ancestor number as well. Founder genome equivalents were initially higher in the GS but decreased over time in both breeds. Generation intervals averaged 23.7 mo in GS and 23.2 mo in LR. Sires had average service lives of 2.7 and 2.2 generations in GS and LR, respectively. Dams had average service lives of 2.1 generations in both breeds. Litter sizes averaged 5.1 and 7.4 pups per litter for GS and LR, respectively. Effective founder and ancestor numbers have slowly increased over time, and heterozygosity as measured by the number of founder genome equivalents in the population has increased. Limitations on the number of matings permitted for sires and service life of dams has led to a plateau for inbreeding and relationships. The importation of germplasm from other working dog colonies is desirable.