According to the national EpiCov survey, conducted on a representative sample of the population, the seroprevalence of SARS-CoV-2, that is, the proportion of people with antibodies against the virus, estimated based on samples collected following the lockdown in May 2020, stands at 4.5% in mainland France among people aged 15 and older. Seroprevalence is highest in Paris (9.0%), in the inner suburbs (9.5%), and in the Haut-Rhin department (10.8%). Living in a densely populated urban area, working in the healthcare sector, or living with a large number of people in the same household are associated with a higher risk of testing positive. Regardless of these factors, the proportion of positive tests is also higher when a household member has had symptoms or tested positive for SARS-CoV-2, further highlighting the role of household transmission in the spread of the virus. Seroprevalence is higher among the 30–49 age group and at the extremes of the socioeconomic distribution. It is also higher among immigrants born outside Europe than among non-immigrants. This is explained by the less favorable living conditions in which some of them live, as this difference disappears when socioeconomic living conditions are taken into account.