ZURICH--Weaker European demand for Switzerland's pharmaceuticals and chemicals, precision instruments and watches, as well as one work day less, eroded export growth in August.

Exports in August fell 3.4% year over year in real terms adjusted for inflation to 14.86 billion Swiss francs ($15.8 billion), the customs office said Thursday. In nominal terms exports rose 0.5%, it said.

Imports reached 13.47 billion francs for a gain of 2.8% in real terms and 4.7% higher in nominal terms.

The trade surplus narrowed to 1.39 billion francs from 3.9 billion francs in July.

Watch exports in August declined a nominal 0.2%, according to the customs office. Watch exports were worth 1.49 billion francs for an increase of 0.7% in real terms.

Write to Neil MacLucas at neil.maclucas@wsj.com

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