First results of a sensitive search for H I envelopes in a complete sample of spiral galaxies: Extensive H I near the isolated Sc I NGC 628

Preliminary results of a search with the Arecibo 1000 foot (305 m) telescope for extended H I envelopes surrounding spiral galaxies show that column densities N(H I)> or =3 x 10/sup 18/ cm/sup -2/ at 2--3 Holmberg radii (R/sub Ho/) are not common. Our sample includes all galaxies of Hubble type later than SO, with optical major-axis diameter greater than 7', and with declination 3/sup 0/--35/sup 0/, and recessional velocity greater than 200 km s/sup -1/.