The country's 43 fund houses together had an average Assets Under Management (AUM) of over Rs 14.22 lakh crore at April-end.
Industry experts said the decline in AUM in May was largely on account of outflows in the liquid or money market category.
Overall outflow in mutual fund schemes stood at Rs 58,185 crore at the end of last month, compared to an inflow of Rs 1.71 lakh crore at April-end, the data by the Association of Mutual Funds in India (AMFI) showed.
Liquid fund mainly invests in money market instruments like certificate of deposits, treasury bills, commercial papers and term deposits and gilt fund only invests in the government securities.
Mutual funds are investment vehicles made up of a pool of funds collected from a large number of investors and invest in stocks, bonds and money market instruments, among others.