Updated from 7:01 a.m. EDTOften, investors look for investment opportunities in large-cap and small-cap stocks, yet they tend to overlook the mid-cap investment opportunities. Standard & Poor's has a MidCap 400 Index, and surprisingly, many of these companies carry fairly high yields. Stockpickr has reviewed the stocks in the index and compiled a list of the Highest-Yielding S&P MidCap 400 Stocks. One of the top-yielding mid-cap stocks is PNM Resources ( PNM), which yields 7.9%. Shares of this distributor of electricity and natural gas have been beaten down recently over rate regulation discussions in New Mexico. But when the Albuquerque, N.M.-based company responded last Thursday to rate-case recommendations by New Mexico regulators, the stock regained some 17%. The stock has a forward price-to-earnings (P/E) ratio of 13 and a P/E-to-growth (PEG) ratio of 1.7. The stockholders of PNM are in good company, as Microsoft's ( MSFT) Bill Gates owns shares of this company through his Cascade Investments investment vehicle. Cascade also owns shares of Canadian National Railway ( CNI), which sports a 2% yield, Republic Services ( RSG), which has a yield of 2.3%, and Grupo Televisa ( TV), which has a 0.7% yield. Hospitality Properties Trust ( HPT) is another high-yield mid-cap stock, with a payout rate of 8.5%. This real estate investment trust specializes in hotels, including Courtyard by Marriott, Residence Inn by Marriott, Staybridge Suites by Holiday Inn and Candlewood Suites. The REIT last month reported a 29% increase in revenue in the fourth quarter. The stock has a forward P/E of 7.5 and a PEG of 1.4. Hospitality Trust shows up in the Stockpickr portfolio called Barron's Research Reports Jan. 19. Other stocks in the portfolio include Boston Private Financial Holdings ( BPFH), which yields 3.6%, Citigroup ( C), with a 5.7% yield, and Frontier Oil ( FTO), which has a 0.7% yield. Lee Enterprises ( LEE), which is also part of the MidCap 400 Index, pays a yield of 6.6%. Lee, which owns more than 50 newspapers throughout the U.S., has a forward P/E of 9.5 and a PEG of 3. Lee appears in the portfolio of the First Trust Value Line Dividend Fund, the first closed-end fund to be converted into an exchange-traded fund. This ETF also holds shares of ConocoPhillips ( COP), which has a 2.5% yield, Telecom Corp. of New Zealand ( NZT), with a 6.9% yield, and Plum Creek Timber ( PCL), which has a 4% yield. To see the entire list of 10 mid-cap stocks, check out the Highest-Yielding S&P MidCap 400 Stocks at Stockpickr.com.