Observations were made 1570 meters of water equivalent underground with an 8000-metric-ton water Cherenkov detector. During a live time of 80 d no events consistent with the decay $p\to e^{+}{\pi }^0$ were found in a fiducial mass of 3300 metric tons. It is concluded that the limit on the lifetime for bound plus free protons divided by the $e^{+}{\pi }^0$ branching ratio is $\frac{\tau }{B}>6.5\mathrm{}\times {10}^{31}$ yr; for free protons the limit is $\frac{\tau }{B}>1.9\mathrm{}\times {10}^{31}$ yr (90% confidence). Observed cosmic-ray muons and neutrinos are compatible with expectations.

• Received 13 April 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.27