Portugal has sold six-month debt with a negative yield for the first time ever, underscoring just how fierce demand for European government debt still is despite the sharp shake up in bond markets this month.

The country, which exited an international bailout program just last year, Wednesday auctioned €300 million ($334.5 million) of Treasury bills maturing in November this year, with an average yield of 0.002 percentage point below zero. In March, the yield it offered at a similar auction was 0.047%.

...