Experimental measurement of the mobility of photoelectrons in low-temperature helium gas is described. At the highest gas densities (near the normal boiling point) the mobility is lower than the value predicted by kinetic theory by four orders of magnitude. At intermediate density a transition region occurs, and at the lowest densities studied, the mobility approaches the kinetic-theory limit. A theoretical discussion of the interaction of a slow electron with a collection of helium atoms is given, and it is shown that at high density and low temperature a correlated ("bubble") state becomes thermodynamically stable. The theory predicts correctly the mobility at high density, the critical density at which the transition occurs, and the approach to the kinetic-theory value at low density. It does not, however, account for the details of the transition region. The observations, and their interpretation, provide strong support for the "bubble" model for electrons in liquid helium. © 1967 The American Physical Society.