A 15-year-old was threatened with a knife on Sunday night in Rockley Road, Luton.
His mobile phone was stolen by a chubby black teenager, around 17, 5ft 10in tall and wearing a blue hooded top and dark jeans.
Investigating officer DC Dave Brecknock would like to hear from anyone in the area at the time of the attack.
He can be contacted in confidence on 01582 394499 or non-emergency number 101. Or ring Crimestoppers anonymously on 0800 555 111.