In another sign of a deepening slump for Oklahoma’s leading industry, gross production tax revenue in March dropped to its lowest level in more than a dozen years, according to data released Monday.

These tax collections from oil and natural gas companies fell to $39.6 million for the month, lowest since September 2002, when $32.5 million was reported, state Treasurer Ken Miller said.

There was a 48 percent drop in this monthly revenue stream compared with March of last year, helping drag overall monthly revenue collections to a decline of $28 million compared with last year.

“It’s a pretty significant drop but certainly not unexpected,” Miller said.