Although it refuses to release viewership totals, Netflix has hyped Stranger Things as a phenomenon. Now, Nielsen has verified that claim.

The ratings service, which began measuring Netflix viewership in August, says 15.8 million American viewers watched the first episode of the series' second season by Sunday, its third day of release. That's slightly ahead of the total for AMC's The Walking Dead over its first-three-days period, but less than the average for NBC's This is Us or ABC's The Good Doctor.

Nielsen also estimates that 361,000 U.S. subscribers watched all nine episodes of Stranger Things 2 last Friday, in a practice that Netflix has labeled "binge-racing."

More:Netflix reveals viewers' favorite 'binge-racing' series: 'Gilmore Girls'

Nielsen says that throughout last weekend, Stranger fans watched an average of 2.9 episodes, and each of the nine episodes drew at least 4 million viewers, with declining totals for subsequent episodes. (Chapter 2 averaged 13.7 million; Chapter 3, 11.6 million; and Chapter 9, 4.6 million for the opening weekend).

Netflix routinely disputes ratings estimates as inaccurate, and in any event Stranger will likely amass far more viewers as the weeks go by.

But Nielsen's measurement constitutes the first from the ratings source for traditional TV audiences.