By Ira Iosebashvili 

The proportion of euros held in official foreign-exchange reserves dropped in the first quarter, suggesting that central banks allowed their allocation of the single currency to decline amid negative interest rates in the eurozone.

Central banks held 21% of their reserves in euros at the end of the first quarter, the lowest share in 13 years, data released Tuesday by the International Monetary Fund showed. That is down from 22% at the end of 2014.

The percentage held in U.S. dollars rose to 64%, from 63% at the end of the fourth quarter.

The data, known as Cofer for Currency Composition of Official Foreign Exchange Reserves, is released on the last business day of every quarter with a six-month lag.

Commanding some $12 trillion, central banks are by far the biggest participants in the foreign-exchange markets, and their actions can drive long-term trends in currencies.

For years, central banks sought to trim their dollar holdings, as the U.S. currency sagged under the Federal Reserve's bond-buying program, which sought to kick-start the U.S. economy by purchasing government securities and lowering interest rates.

The shift out of the euro comes after the European Central Bank cut interest rates to below zero a year ago for the first time, in a bid to stave off recession. Forced to pay for the privilege of owning euros, central banks have been trimming their share of the European currency, with much of those funds finding their way into dollars.

In March, the ECB kicked off its own large-scale bond-purchasing program, adding to the pressure on its currency.

The euro fell as low as $1.0462 in the first quarter, its weakest level in more than 12 years, lowering the valuation of reserves held in the currency. While central banks would historically use dips in the euro as an opportunity to buy, "it seems that global reserve managers continue to have 'portfolio rebalancing' on hold in the face of monetary-policy divergence and negative interest rates," said Jens Nordvig, managing director at Nomura, in a note to clients.

Interest rates in Europe have crept higher since March amid improving economic data on the Continent, buoying the euro.

In late New York trading Tuesday, the European currency was at $1.1141.

"The question at this point is whether or not this trend continues in light of the fact that European interest rates have rebounded," Mr. Nordvig said.

At the same time, some investors have grown uncertain about whether the dollar will resume a rally that saw it rise by its sharpest level in 40 years between July 2014 and March 2015--a time during which central banks increased their allocations of the U.S. currency.

While the Fed is expected to tighten monetary policy for the first time in nearly a decade this year, some market watchers believe that an uneven U.S. economy will keep the central bank from raising borrowing costs very far.

The Wall Street Journal Dollar Index, which measures the buck against a basket of 16 currencies, closed at 86.60 Tuesday, down 3.1% from its March high.

Write to Ira Iosebashvili at ira.iosebashvili@wsj.com