The tax penalty for noncompliance with the Affordable Care Act's individual mandate is to be eliminated starting in 2019. We investigated the potential impact of this change on enrollees' decisions to purchase insurance and on individual-market premiums. In a survey of enrollees in the individual market in California in 2017, 19 percent reported that they would not have purchased insurance had there been no penalty. We estimated that premiums would increase by 4-7 percent if these enrollees were not in the risk pool. The percentages of enrollees who would forgo insurance were higher among those with lower income and education, Hispanics, and those who had been uninsured in the prior year, relative to the comparison groups. Compared to older enrollees and those with two or more chronic conditions, respectively, younger enrollees and those with no chronic conditions were also more likely to say that they would not have purchased insurance. Eliminating the mandate penalty alone is unlikely to destabilize the California individual market but could erode coverage gains, especially among groups whose members have historically been less likely to be insured.