To answer the question, you basically need to know what an ultrasonic repeller is. Ultrasonic technology is simply a modern means of producing and emitting the high frequency soundwaves. And, an ultrasonic repeller is a device, built in with the ultrasonic technology, to be able to produce the soundwaves or vibration at a high frequency which animals or pests can hear but not humans. 

An ultrasonic pest repeller is able to get rid of the pests at home in a way that it continuously produces a high frequency soundwave which is going to be so annoying and disturbing to the hearing of the pests. That is to communicate a message that they can not stay around the frequency zone. If the particular ultrasonic pest repeller could produce the type of high frequency that can really annoy the pest you are attempting for, it will work to chase them away. And, different pests might share the same or different frequency range. The simple but whole story about the ultrasonic pest repeller is that simple.

However, the answer to the question of if the ultrasonic repeller works on fleas is to the same story. It is about if the ultrasonic repeller you are having works particularly to the fleas which you want away. It then comes to the question of how you can find the ultrasonic repeller that works with fleas. Generally, fleas are another common type of pests, and it shares the same frequency range as many others.

Because many pests share the same frequency range, a lot of ultrasonic repellers on the market have been advertised that it works with most of the common pests. It generally is so, and you would see those pests by names, including mice, mosquitoes, ants, spiders, roaches, bugs and more. What they actually mean is their repellers work with these common pests.

In case, it is the general pests like that you are trying to deal with at home, you are good to go for one of the best reviewed ultrasonic pest repellers, and it should be working alright for you. Many of them are available while the ones with most positive reviews are reliable ones by actual user experience. Nevertheless, if it is the particular fleas you are really wanting to deal with and chase away from your pets, you can go for a more detailed search for the kind of repeller.

Of course, there will be certain ultrasonic repellers that mention the fleas in their list of the pests their ultrasonic repellers will work with. That could somehow ensure you a bit more. But, it, at the same time, is going to take your time a little bit more. Instead, to the generality, a good ultrasonic technology in an ultrasonic repeller should be working great to fleas as well as other common pests many households are facing. 

By the way, an ultrasonic repeller can easily be found while available at a very affordable price. Though it could be the worse case that the repeller does not unfortunately work for you, it will not be a big deal. That could mean you can basically go get it and have the actual testing. It might save your time even more.