Mendel's Accountant is a user-friendly, biologically-realistic forward-time population genetics simulator that provides opportunity to investigate the effects of various factors in diploids on population fitness and allele loss over generations (see abstract for “Introducing Mendel's Accountant…” in section A1). Determining minimum population sizes required to preserve genetic diversity in seed banks is critical to maintaining sufficient sample numbers within resource constraints.  Mendel’s Accountant allows investigation of allele loss and mutational changes as a function of population size, frequency of seed increase, natural or artificial elimination of the least-fit individuals, and various other factors.  While other forward-time population genetics simulators exist, Mendel's Accountant excels in ease of use, graphic output, biological realism and applicability to problems of genetic resource preservation. Mendel's accountant can be downloaded free of charge at http://sourceforge.net/projects/mendelsaccount. It has only recently been made publicly available, and we welcome suggestions for improvements and reports of experiences with its use in a teaching or research context.