Utilizing a data file extracted from the University of Michigan Survey Research Center's Panel Study of Income Dynamics, this study examines the work experiences of a national sample of non-aged heads of households. This data source, which describes heads of households and structural characteristics of labor markets, is unique in that underemployed heads of households indicate whether their part-time working status is voluntary or involuntary. Among the heads who were employed during 1971, 15% were part-time workers in the sense that they worked, on average, less than 30 hours per week and/or no more than 40 weeks per year. Findings of this study indicate that 54% of these part-time workers were involuntarily underemployed, willing but unable to devote more time to gainful employment. Furthermore, household heads who were involuntarily underemployed in 197l constituted a clear majority of all non-aged heads who were (1) unemployed (58.0%) and (2) recent labor force dropouts (55.5%) during the spring of l972. These heads of households are, when working, concentrated in low wage jobs. (Author)