Two teenagers have been arrested and reprimanded by police for "cyber bullying" on a young people's website.
A survey found one in ten youngsters has been bullied online
Dyfed-Powys Police said the pair from Haverfordwest posted pictures of people on Bebo and added offensive comments.
Officers warned they will do the same in similar cases and said the culprits could be left with a criminal record.
Anti-bullying charity Kidscape said web bullying was rising and praised the police response, and Bebo said it was "leading the field" against bullying.
Sergeant Jill Evans said the two youngsters had posted the pictures and comments on the US-based social networking site Bebo.
"Bullying is not acceptable no matter what form it takes," she said.
Police said a reprimand for juveniles is similar to a caution for adults and means the youngsters will have a police record.
Sgt Evans added: "We will treat any report of bullying seriously and we are appealing to young people to act sensibly on the internet and for parents to monitor them else they could end up with a criminal record."
In a MSN/YouGov survey last year 10% of UK teenagers said they had been bullied online while 24% said they knew a victim.
Nikki Kerr of Kidscape said: "The whole idea of cyber-bullying is on the rise and children are using new technology to cause harm, distress and bully other children.
"Sometimes it can be even worse when it happens than in the real world because it can be so anonymous, yet so many people can be a witness to it by using the internet."
She said because it was such a new and evolving form of bullying there was no standard way for police to deal with offenders.
But she welcomed the response by Dyfed-Powys Police.
"It's a good thing if it shocks those responsible into realising the distress they can cause," she added.
Last year Bebo announced that it had put new safety features in place.
In September 2006 Bebo said it was "leading the field in terms of combating bullying on its service".
It said it would allow users to vet comments and images others tried to post on their personal homepages before they became visible. Users could also block others from interacting with them.
The website said these tools were an "incredibly important development and will have a significant impact in terms of reducing the public humiliation aspects of online bullying."
Bebo said it also worked with a variety of organisations, including anti-bullying groups, the Home Office internet task force on social network, and the Department for Education.