Fewer than half of 15-year-olds reach GCSEs standard
Fewer than half of 15-year-olds are getting five or more GCSEs at A* to C grade, including English or Welsh first language and maths.
The proportion reaching the standard rose slightly from 49.4% to 49.6% this year.
Figures published on Thursday showed a slight improvement in teenagers' exam results.
But the Welsh government said a slow-down in improvement showed there was more to do to drive up standards.
It is not yet possible to compare this year's performance with England as results over the border have not been published.
However, the equivalent figure in England last year was 53.1%.
Nine in 10 (89.9%) Welsh 15-year-olds got the equivalent of at least five GCSEs at grades D to G this year.
Those achieving no recognised qualification fell to its lowest level since 2001/02 - 0.7%.
Provisional figures from the Welsh government show the number of 15-year-old pupils fell from 37,072 to 36,088.
The 0.2% improvement in the proportion reaching the equivalent of five or more GCSEs at A* to C, including core subjects - what the Welsh government calls its headline indicator - is smaller than previous years.
A Welsh government spokesman said the slow-down confirmed Education Minister Leighton Andrews was right to call for more to be done.
Mr Andrews has denied that a banding system he is introducing for comprehensive schools will name and shame the worst performers, but will help make sure resources go to where they are needed most.
The spokesman said: "We welcome the improvements shown in these provisional results.
"However, the slow-down in rates of improvement against our headline indicator reaffirms the minister's view that there is much more to be done.
"We need to ensure that we see continuous improvement in the literacy and numeracy skills of our young people."
He added: "The final data will feed into our new banding system for secondary schools and help us see where our strengths and weaknesses lie.
"The minister has already set out a challenging agenda for improvement; we must remain focused on the tasks in hand."
Plaid Cymru education spokesman Simon Thomas said he was concerned that Labour had not included targets for education standards when it published its programme for government last month.
He said: "If the Labour government and minister are serious about improving education standards they should publish targets so they can be held to account."
Welsh Liberal Democrat education spokesman Aled Roberts said GCSE results needed to be better.
"If the current trend in Welsh and English results continue, I suspect that when the English figures are published, the gap will have increased yet again," he said.