diff --git a/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.html b/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.html
index 9e4b8c1..7ca52ec 100644
--- a/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.html
+++ b/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.html
@@ -14,6 +14,9 @@
           <div class="error-message" *ngIf="username.errors.required">
             This field is required.
           </div>
+          <div class="error-message" *ngIf="username.errors.pattern">
+            Username must contain only lower case letters and numbers.
+          </div>
           <div class="error-message" *ngIf="username.errors.minlength || username.errors.maxlength">
             Username must be between 3 and 50 letters.
           </div>
diff --git a/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.ts b/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.ts
index ed84eff..4dda8e5 100644
--- a/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.ts
+++ b/client/app/experiment-participants/create-user-account-dialog/create-user-account-dialog.component.ts
@@ -54,7 +54,7 @@ export class CreateUserAccountDialogComponent implements OnInit, OnDestroy {
 
   password = new FormControl("", [Validators.required, Validators.minLength(4)])
 
-  username = new FormControl("", [Validators.required, Validators.minLength(3), Validators.maxLength(50), noDuplicateValidator(this.data.participants.map(p => p.username))])
+  username = new FormControl("", [Validators.required, Validators.minLength(3), Validators.pattern('^[a-z0-9]+$'), Validators.maxLength(50), noDuplicateValidator(this.data.participants.map(p => p.username))])
 
   email = new FormControl("changeme@email.com", [Validators.required, Validators.email])